Chesapecten middlesexensis bayshorensis Waller, 2018
WALLER, T. R. 2018. Systematics and biostratigraphy of Chesapecten and Carolinapecten (Mollusca: Bivalvia: Pectinidae) in the upper Miocene and Pliocene "lower Tamiami Formation" of southwestern Florida. Bulletin of the Florida Museum of Natural History, 56 (1): 1-47, figs. 1-11. [p. 14, figs. 5A-5G]
2018 Chesapecten middlesexensis bayshorensis Waller, 2018
T. R. Waller, 2018, figure 5.
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«Zoobank Nomenclatural Act.—F8BF0579-FB5C-4DBA-8EFF-9CF149C32E94.
Diagnosis.— Moderately thick-shelled, slightly left-convex Chesapecten middlesexensis with 6−8 radial ribs, rarely nine or ten, with coarse radial scabrous costae on ribs and interspaces; byssal notch remaining deep with active ctenolium throughout ontogeny; byssal fasciole slightly sunken, lacking radial costellae; hinge dentition weakly developed. Description.—Shell of moderate size, reaching heights of about 80−90 mm, equilateral to slightly posteriorly extended, with length and height about equal in early ontogeny but length exceeding height in late ontogeny; both valves moderately convex, with left valve more convex than right and with narrow disk gapes; byssal notch remaining deep with active ctenolium throughout ontogeny; byssal fasciole slightly depressed, lacking radial costellae; byssal sinus of left valve shallow. Disks with 6–8 radial ribs, rarely 9 or 10; ribs high, with slightly rounded crests and steep, vertical sides in early ontogeny, becoming lower and broader with sloping sides in late ontogeny; ribs, interspaces, and disk flanks covered by coarse radial costae of uneven strengths beginning in interspaces of central sector at shell heights of 12–15 mm and increasing in number by intercalation, numbering 5–9 in interspaces on right valve and 8–11 on left valve at shell height of 50 mm. Total hinge length about 60% of shell length, with anterior hinge slightly longer than posterior hinge; byssal notch remaining deep throughout ontogeny, with apex of notch forming an acute angle; active ctenolium continuing to form throughout ontogeny; byssal sinus of left anterior auricle shallow, with overall trend of left anterior auricular margin forming an acute angle with dorsal margin; margins of posterior auricles nearly straight or forming slightly acute angle with dorsal margin in early ontogeny, becoming perpendicular to or forming slightly obtuse angle with dorsal margin in late ontogeny; all auricles densely and finely costate, with number of costae ranging from about 12–18 at margin of right anterior auricle and 16–27 at margin of other auricles. Weak dorsal hinge teeth and resilial teeth present throughout ontogeny. Auricular denticles well developed at distal ends of ridges on shell interior at ventral edge of auricles. Pallial line inset far from shell margin at about two-thirds height of shell. Umbonal inner foliated calcite layer extending ventrally to the mid-adductor-scar level, not extending beneath posteroventral margin of adductor scar. Etymology.— Named with reference to the type locality on the Bayshore Waterway, Charlotte County, Florida. Type material and measurements.— Holotype: USNM 716588, right valve, Ht 82.0 mm, L 89.0 mm, convexity 16.4 mm (Fig. 5A–D). Type locality.— USGS 23858, Charlotte County, Florida; spoil banks of Bayshore Waterway on north side of Rte. 41, about 5.1 km (3.2 miles) west of junction of U.S. 41 and Rte. 77. T. R. Waller, July 11, 1962. Other material.— About 100 specimens (single valves plus one pair of matching valves) from 11 localities in the Port Charlotte area. Comparisons.— Chesapecten middlesexensis bayshorensis differs from Ch. middlesexensis hunterae in having fewer ribs (commonly 6–8, compared to 10 or 11), a deeper byssal notch that remains deep throughout ontogeny, and a ctenolium that persists throughout ontogeny. Compared to Ch. jeffersonius and Ch. septenarius, Ch. middlesexensis bayshorensis has a deeper byssal notch, a more differentiated non-costellate byssal fasciole, and rib profiles that are less squared throughout ontogeny. Compared to variants of Ch. madisonius sarasotensis n. ssp. that have only seven or eight ribs, Ch. middlesexensis bayshorensis differs in having a deeper byssal notch, a more differentiated byssal fasciole throughout ontogeny, and a thinner, less biconvex shell. Chesapecten quinarius differs from Ch. middlesexensis bayshorensis in having fewer ribs (commonly only five or six) that are more rounded in profile throughout ontogeny. Occurrence.— Port Charlotte area, west of Murdock: Auburn Waterway: USGS 22256, CH104A; Como Waterway: USGS 22931; Crestwood Waterway: USGS 22916; Bayshore Waterway: USGS 26931, USGS 26932, USGS 23858 (type locality), USGS 26910, USGS 26911. Port Charlotte area, east of Murdock: Sunset Waterway: USGS 26915. Distribution.— Known only from the Port Charlotte area in the southwestern Florida Peninsula, where it is apparently limited to Hunter’s (1968) lower Murdock Station Member, now regarded as within the upper Peace River Formation of late Miocene (Tortonian) age. In the present study, Ch. middlesexensis bayshorensis is assigned to pectinid zone PZ3 (see Biostratigraphy section). Remarks.— Chesapecten middlesexensis bayshorensis is present at several localities along the Bayshore Waterway in the Port Charlotte area, where it co-occurs with Ca. murdockensis parawatsonensis, n. ssp. as well as with the late Miocene barnacle Chesaconcavus myosulcatus (USGS 26931) and non-auriculate oysters of the Mansfieldostrea compressirostra complex (USGS 26910).» THOMAS RICHARD WALLER, 2018
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