Sectipecten allani Marwick, 1928
MARWICK, J. 1928. The Tertiary Mollusca of the Chatham
Islands including a generic revision of the New Zealand Pectinidae.
Transactions and Proceedings of the New Zealand Institute, 58 (4): 432-506. [p. 459, figs. 13, 21]
1928 Sectipecten allani Marwick, 1928
J. Marwick, 1928, figures 1-13.
J. Marwick, 1928, figures 14-21.
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«Shell large, strong, subcircular, apical angle obtuse, both valves inflated, right more than left. Ears unequal, anterior sinus shallow. Sculpture somewhat variable; type has in right valve 9 broad flat ribs, separated by interstices about half their width, the ribs have vertical sides and so their, cross section is quadrangular, on their surfaces are faintly impressed from foul' to six broad secondary ribs. Interspaces have one strong narrow median rib throughout nearly the whole length, with weak one on each side coming in later. On anterior and posterior distal portions of disc are about five narrow radial riblets. Whole surface is covered with fine regular concentric ridges which in rib interspaces and on ears become much stronger and sharply raised, posterior ear with three obsolete, widely-spaced radials, anterior with 4 strong spaced radials and some secondaries. Left valve with 9 rather narrow, strong, rounded primary ribs which are weakly divided from about one inch from apex, some ribs have indications of still further division of each half. Interspaces arectwice as wide as ribs, corresponding to the ribs of right valve; eachchas from 3 to 4 strong, rounded, secondary riblets, whole surfacecincluding ribs and ears covered by dense, sharp raised concentric growth-ridges; each ear with about 3 obsolete radial threads.
Height 98 mm.; length 104 nun.; thickness of right valve 13 mm.; left valve 10 mm. Locality: 1176, Momoe-a-toa. Remarks: This is the largest and probably. the commonest shell in the Pecten bed at this place. Some specimens reach 120 mm. in diameter. The inflated specimens have generally strong primary but weak secondary ribbing. Other shells have on the right valve rather weak primaries which are divided almost up to the apex, one or both limbs being grooved. These variations approach Sectipecten wollastoni Finlay (= sectus Hutt.); but in no case is strong primary ribbing accompanied by deep secondary grooving. The two species are closely related, but S. allani seems to be the less advanced, for the inflated adults are at about the stage represented by S. wollastoni of 30 mm. diameter.» JOHN MARWICK, 1928
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«Moderately large for family (90-110 mm high), length equal to height in small specimens and in finely costate large shells, but length markedly greater than height in many coarsely costate large specimens; left valve slightly more inflated than right. Ears almost equal in 2 valves, nearly symmetrical; anterior margin of anterior ears sinuous, with broad, shallow byssal sinus but no ctenolium in right valve. Sculpture of 8 to 11 (on most specimens, 9 or 10) wide, flat-topped, square-edged radial folds on right valve and a similar number of high, narrow, widely spaced folds on left valve of coarsely sculptured end-members, with 2 or 3 low, square-edged, secondary costae in each interspace; a similar number of lower, narrower, medially subdivided folds tending to fade out towards ventral margin and with more numerous lower, flat-topped secondary costae on right valve of more finely sculptured end-members; right valve of younger (Waipipian-Mangapanian) forms develops a deep subdividing groove down the centre of major costae, and corresponding central narrow costa in wide interspaces on left valve.
Opoitian-Mangapanian; Momoe-a-Toa Shellbed, Cape Young, northern Chatham Island, early Opoitian (type); Opoitian specimens are known from a few mainland localities (Kaawa Creek, southwest Auckland; limestone, Dyerville road, northern Aorangi Range, southern Wairarapa), Waipipian specimens are known from equally few localities (Black Reef Sandstone, Cape Kidnappers; conglomeratic shellbed near head of Greek's Creek, Arahura Valley, Westland) and specimens are not uncommon at one Mangapanian locality (rubbly limestone in Blythe Valley, North Canterbury; with Phialopecten triphooki). The type population of Sectipecten aIlani is very complex, and fortunately thousands of specimens can be observed on an extensive bedding plane; the fine and coarse endmembers are linked by a complete range of intermediates, and range from shells resembling S. grangei (Tongaporutuan; PI. 28c) to extreme, flat-ribbed S. allani; as shown under Sectipecten mariae (PI. 38h, i), the fine end-members apparently gave rise to S. mariae. The coarsely costate end-members developed into a slightly more subdivided form (Opoitian-Mangapanian of mainland New Zealand) occurring only rarely in barnacle limestone, conglomerate, and coarse, near-shore sandstone, and almost all known specimens (other than at Momoe-a-Toa) are undiagnostic left valves. Some relatively simple right valves are known from a Mangapanian limestone in the Blythe Valley, North Canterbury, and a single excellent articulated specimen from a conglomeratic shellbed in Greeks Creek, Arahura Valley, Westland (Waipipian; Otago University Geology Department, OU11904) shows that advanced right valves have the major folds subdivided by a deep groove, a new character in the lineage. The main Sectipecten lineage clearly became adapted to progressively more near-shore, high-energy environments after Opoitian time, and its place in limestone facies appears to have been taken by S. mariae. Figured specimen (PI. 33c): GS12184, CH/f2C, Momoea- Toa Shellbed, northern Chatham Island, early Opoitian (NZGS) x 0.67.» BEU, A. G. & P. A. MAXWELL. 1990. Cenozoic Mollusca of New Zealand. New Zealand Geological Survey Paleontological Bulletin, 58: 1-518, pls. 1- 57. [p. 276, 278]
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Sectipecten allani Marwick; A. G. Beu & P. A. Maxwell, 1990, Cenozoic Mollusca of New Zealand, plate 33, figure c.
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«DISCUSSION. Sectipecten allani is represented in the Geological Survey collection by 12 well preserved specimens from Momoe-a-Toa, including single valves and double-valved individuals, which show a continuous range of variation in sculpture. Two right valves have 8 or 9 broad, strong, inflated, fold.like ribs, entire and smooth-surfaced from apex to ventral margin, and markedly wider than their interspaces. Interstitial riblets are almost obsolete and appear only at 65 to 70 mm from the apex. Corresponding left valves have the same number of primary ribs separated by very broad interspaces in which riblets are nearly obsolete. These forms are considered to have the most advanced and specialised type of sculpture. Four more right valves, including the holotype, have broad ribs that are lower and less convex and show faint grooving starting at about 30 mm from the apex. Another specimen has relatively narrower right valve ribs with more deeply incised grooves that start earlier. Finally, two right valves and a left valve have the least specialised sculpture. Primary ribs are low, particularly towards the ventral margin, and on the right valve are deeply split by a median groove initiated almost at the apex. On one specimen a faint groove appears later on one limb of the divided rib. On the other specimen no further grooves appear.
In discussing the possible derivation of S. allani the obvious comparison is with S. wollastoni from the Kapitean of the mainland. Both species have broad fold-like ribs on the right valve and corresponding wide interspaces and narrow ribs on the left valve. On S. wollastoni the folds are entire on the earlier part of the disc, but grooves begin at about 25 to 40 mm from the apex, whereas on the most advanced forms of S. allani the folds extend without division to the ventral margin of the shell. These forms of S. allani are closely comparable to a single right valve from the Opoitian at Kaawa Creek, which is probably a mainland development from the Kapitean S. wollastoni. Less advanced forms of S. allani (such as the holotype) are not very different from S. wollastoni, particularly when left valves are compared; right valves are superficially dissimilar because on S. wollastoni the major ribs are deeply grooved, whereas on S. allani the grooves are partially suppressed. The least advanced forms of S. allani have no counterpart in populations of S. wollastoni except perhaps for one small shell from Kapitea Creek (GS 2888) that is considered to be an early, primitive form of S. wollastoni (see above, p. 664).» BOREHAM, A. U. E. 1961. The New Zealand Tertiary Genus Sectipecten Marwick (Mollusca). Transactions of the Royal Society of New Zealand, 88 (4): 655-668, pls. 45-49. [p. 665]
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Sectipecten allani Marwick; A. U. E. Boreham, 1961, The New Zealand Tertiary Genus Sectipecten Marwick, plate 49, figures 13, 14.
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