Pecten imbrifer var. minor Jensen, 1912
JENSEN, A. S. 1912. Lamellibranchiata. Part 1. With and Introduction on Greenland's Fossil Mollusc-fauna from the Quaternary Time. Danish Ingolf-Expedition, 2 (5): 1-119, pls. 1-4. [p. 25, figs. 2a, 2b]
«Pecten imbrifer Lovén.
Pl. II, fig. 1 (var. maior) & fig. 2 (var. minor). Pecten imbrifer Lovén, Index Moll. Scand., 1846, p. 31; Dall, Bull. Mus. Comp. Zool. Harvard Coll. XII 1886, p. 220, PI. IV, fig. 4; Friele & Grieg, Norw. North-Atlantic Exped., Mollusca III, 1901, p. 8. — Pecten mamillatus M. Sars, Christ. Vid. Selsk. Forh. 1873, p. 12. — Pecten Hoskynsi G. O. Sars, Moll. Reg. Arct. Norv., 1878, p. 20, PI. 2, fig. 1; var. major Leche, K. Sv. Vetensk. Akad. Handl. 16, No. 2, 1878, p. 35; Collin, Dijmplma-Togtets zool.-bot. Udbytte, 1886, p. 453; var. pustulosus Verrill, Trans. Conn. Acad. V, 1882, p. 581, PI. 42, fig. 22. — Cyclopecten pustulosus Verrill, Trans. Conn. Acad. X, 1899, p. 70, fig. 1 &: p. 83, Pl. 19, figs. 3, 4; Verrill & Bush, Proc. Unit. Stat. Nat. Mus. XX, 1898, p. 839, PI. 85, figs. 5, 6, 10, 11. — Cyclopecten subimbrifer Verrill & Rush, Trans. Conn. Acad. X, 1899, p. 84; Proc. Unit. Stat. Nat. Mus. XX, 1898, p. 840, PI. 85, figs. 8, 9. — Pecten lucidus Nomann, Niederländ. Arch. f. Zool., Suppl. Bd. I, 1881—82, No. 10, p. 2, Pl. 1, figs. 5, 6, 7, 8.
Pecten Hoskynsi Friele, Nyt Mag. f. Naturvidensk. 24 Bd., 1879, p. 222; Becher, Österr. Polarst. Jan Mayen, 1886, III, p. 68. — Pecten imbrifer Posselt, Medd. om Grønland, XIX, 1895, p. 66; ibid. XXIII, 1898, p. 12; Jensen, ibid. XXIX, 1905 (1909), p. 332; Hägg; Arkiv för Zoologi, Bd.2, 1904, No. 2, p. 30; var. lamellosa Posselt, Medd. om Gronland, XXIII, 1898, p. 13, Pl. 1, fig. 1. Pecten imbrifer has been taken by the "Ingolf" at:
West Greenland.
During my cruise with the "Tjalfe" I took P. imbrifer in Davis Strait at:
66°22' N.L., 57°16' W.L .......... 360 fm 6 spec. The Swedish Expedition of 1871 found it in Umanak Fjord at 397 fm. and in Baffins Bay at 227 fm. East Greenland.
Danish Expeditions have taken P. imbrifer at the following places:
Off Scoresby Sound ........................... 167 fm. 1 valve. Forsblads Fjord .................................. 50 - 1 -- — — .................................... 50-90 - ca. 125 spec. The Swedish Expedition of 1900 took P. imbrifer at:
S. E. of Pendulum Island (74°35' N.L.) ....... 79 fm. 1 spec. Jan Mayen.
In addition to at the above stations of the "Ingolf", P. imbrifer has been taken by the Norwegian North-Atlantic Exped. at a depth of 263 fm. (bottom-temp. — 0.3º C.) and by the Austrian Exped. at a depth of 75—95 fm.
Iceland.
In addition to at the 8 "Ingolf" stations, P. imbrifer has been taken by the "Thor" at:
63°05' N.L., 20°7' W.L. ............... 293 fm 1 spec. and 10 valves. 62''57' — 19°58' — ................ 500 - 1 — - 9 -- The Faeroes.
As well as N. of the Faeroes, as noted above, P. imbrifer has been taken (by the "Thor") of the Faeroes, at:
61°15' N. L., 9°35' W. L. ......... ca. 475 fm. 12 spec. and 25 valves. Remarks. The specimens to hand of Pecten imbrifer from East Greenland and from Jan Mayen differ from the Atlantic specimens (W., S. W. and S. of Iceland, as well as S. W. of the Faeroes) in attaining to a greater size, namely 15—22 mm, for full-grown specimens, against 10—12 mm in the Atlantic, as also in the fact, that the posterior ears of the shell are relatively large (cf. in Pl. II figs.
1 a, b, c, d, e and f with figs. 2 a and b). On the whole the Polar Sea form probably reaches a greater size than the Atlantic. The specimens brought home by the Dijmphna Exped. from the Kara Sea measure up to 21 mm., and Leche records the maximum size from the Kara Sea as 22 mm. ¹); Friele and Grieg give 20 mm. as being not uncommon for high-arctic specimens, whereas the species does not become more than 11 mm. at the Norwegian coast. Leche for his specimens found it convenient to set up a var. major; I quite agree with this and propose therefore to call the southern, smaller form var. minor. So far as my experience goes, the variety major also differs from the more southerly form by having as a rule the posterior ears of the shell relatively large (comp. in PI. II figs, 1 a—f (var. major) with figs. 2 a—b (var. minor)). I am of opinion, therefore, that the forms major and minor represent geographical subspecies, connected respectively with the "cold" (or transitional belt to this) and the "warm" area in the deeper regions. ²) Within each of these subspecies there are numerous modifications in regard to the sculpture of the left valve. Posselt (I.c.) has some pertinent remarks on this point. Whilst Posselt received his impressions from "a consideration of the figures cited", I have been able to observe the transitions on the material before me and I may take the opportunity of illustrating these not uninteresting conditions by some figures. The lines of growth on the right valve appear as numerous, low and sharp folds, whilst the left valve, in addition to similar folds, bears in general radially arranged, pored, small vesicles (PI. II, figs, 1 g — h), the number of which increases towards the periphery, new ones arising regularly in the interspaces. Closer consideration shows, that the vesicles have the lower edge free and that they are simply outgrowths of the concentric lamellae. Thus, we may find some specimens, in which the left valve in regard to sculpture only differs in the main from the right by some few, distant rows of very small, pocket-like outgrowths (PL II, fig. 1 a), but generally their number is large (fig. 1 b). Other specimens have larger and fewer, rather vaulted outgrowths (fig. 1 c). In others again the vesicles are so broad and flat, that they almost meet one another (fig. 1 e). Lastly, we find in the variety lamellosa Posselt (fig. 1 f) a form where the vesicles are fused together to concentric, porous wrinkles, formed of projecting, down-turned lamellae, which at their lower free margin reach to and rest on the next wrinkle; the lamellae are in general interrupted and broken, so that only remnants of them remain as sharp combs. The vesicular outgrowths are also broken off more or less and their position is then shown as a concavity in the line of growth; sometimes almost all the vesicles are rubbed off, so that the lines of growth appear like the cogs on a cog-wheel (fig. 1 d &. 1 i). Among other variations it may further be mentioned, that the upper part of the shell may be almost completely smooth and the outgrowths appear only towards the margin, as shown in fig. 2 a; lastly, fig. 2 b represents a specimen in which the left valve is almost quite smooth. I would not have entered so much into detail on these points had not Prof. A. E. Verrill and Miss Katherine Bush raised these variations to the rank of species; their Pecten pustulosus and P. subimbrifer are in fact based on specimens such as are represented in my figures 1 c and 1 d. Distribution. The form major is an arctic, deep-water form (30—400 fm., perhaps even deeper, 650 fm. ³), occurring at East Greenland, Jan Mayen and Spitzbergen, in the Barents Sea and the Kara Sea ⁴). The form minor is an Atlantic, deep-water form, which occurs in Davis Strait and off the east coast of the United States of North America, down to ca. 40°N. L., along West and Soutli Iceland, S. of the Faeroes and at the western and northern coasts of Norway. The "Thor" has taken it S. E. of the Faeroes (61º25' N.L., 4°39' W.L., 210 fm.) and in the North Sea (58°32' N.L., 4°18' E.L., 147 fm.). How far the species goes southwards along the west coast of Europe cannot be stated ⁵), as P. imbrifer is said not to be identical — as assumed earlier — with the Mediterranean-Atlantic P. Hoskynsi Forb. ⁶).»
¹) In the Vega-Exped. Veteusk. lakttagelser III, 1883, p. 452 Leche records the species from the Bering Sea with an even greater size, namely 30 mm., but the specimens in question I have had the opportunity of seeing in the Riks-Museum at Stockholm, do not belong to this species.
²) The specimens from St. 59 and St. 143 of the Ingolf, with negative temperatures, are small but seem, to judge fom the relatively large, posterior ears of the shell, to be the young of var. major. ³) St. 192 of the Norvveg. North-Atlantic Exped. lies at 69º46' N.L., 16°15' E. L., with a depth of 649 fm. and bottom temp, of —0.7° C, but I have not seen specimens from here. ⁴) Leche's record of its occurrence in the Bering Sea is due to an erroneous determination, as shown on p. 26 note 1. ⁵) Dautzenberg & Fischer record it right down at the Azores (Dragages effectues par I'Hirondelle et par la Princesse Alice; Mem. Soc. Zool. France 1897, T. 10, p. 192). ⁶) Cf. Dall, Bull. Mus. Comp. Zoology, XII, 1SS6, pp. 214 and 220. ADOLF SVERIN JENSEN, 1912
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A. S. Jensen, 1912, plate 2.
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