Bairdoppilata thyreoides Maddocks, 2025, sp. nov.

Bairdoppilata thyreoides sp. nov. (Graphs 5, 6; Figures 2N–O; 30A–T; 31A–S; 32I–K; 33J–K) Material Studied: 45 specimens. Derivation of Name: New Latin thyreoides, third-declension, one-termination adjective; from ancient Greek θυρεοειδής, shield-shaped; in reference to the shape of the central opaque patch. Types: Holotype specimen 4180 LV, USNM 1751331. Paratype specimens 4153 RV, USNM 1751332; 4154 RV, USNM 1751333; 4179 LV, USNM 1751334; illustrated specimens USNM 1751335–1751338. Type Locality: Bahamas, UH 2376. Off Harbour Lodge Beach, Hope Town, Atlantic Ocean side (east side). 26 o 32’N, 76 o 57’W. On Elbow Cay, east side of Great Abaco Island, coral reef, water depth 30–35 feet; 100 yards offshore, sandy bottom, non-turbid, within living reef growing on large limestone blocks. Occurrence: Bahamas. Dimensions: Holotype specimen 4180 LV, LV L 0.972mm, LV H 0.567 mm. Paratype specimen 4179 LV, LV L 0.891 mm, LV H 0.509 mm. Paratype specimen 4153 RV, RV L 0.876 mm, RV H 0.468 mm. Paratype specimen 4154 RV, RV L 0.914 mm, RVH 0.493. Paratype specimen 4184Car, L 0.833 mm, W 0.356 mm. Paratype specimen 4185Car, L 0.872 mm, W 0.386 mm. See also Graph 5. Diagnosis: Carapace long, moderately high; LV with shield-shaped central opaque spot, dorsal spots separate or connected to central spot; anterior and posterior opaque spots very wide and vertically extended; lateral fields moderately transparent. Description: Carapace outline elongate-oval, with high, nearly symmetrically domed dorsal margin, moderate overreach, steep anterodorsal and posterodorsal margins, level ventral margin, and obliquely curved anteroventral and posteroventral margins (Figures 29B–C; 30F–G). Caudal process conspicuous, straight, slightly swollen but not ridged, and set at about one-third height (Figures 29A–E; 30P). Dorsal and ventral outlines slender, with curved contours, tapering to narrow anterior angle, and abruptly constricted, especially in LV, just in front of narrowly produced posterior end (Figure 30A–D). Valve edges tightly compressed at anterior and posterior ends, not flared. Opaque patch pattern shows intense contrast (Figure 29A–E); shield-shaped central streak of LV has symmetrically sloping dorsal edges, tapers downward to ventral margin; two large dorsal spots connected in Ypattern or separated by cloudy region. Lozenge-shaped central spot of RV narrows downward, almost to ventral margin; thinner streak may connect to dorsal margin. Anterior and posterior spots large and dense, triangular, broad and tall, extended dorsally, ventrally, and horizontally. Hinge unremarkable (Figure 30J). Bairdoppilatan supplemental dentition present, but RV denticles tiny, hard to see (Figure 30N–O). ZC wide with shallow vestibules (Figure 29A–M). Selvages curved, close to margins; flanges narrow (Figure 29H–N). LV anteroventral margin smooth or with fragments of a frill, posteroventral margin smooth; RV anteroventral and posteroventral margins smooth (Figures 29F–N; 30F–M). Juvenile A–1 LV anteroventral margin with fragments of a frill; posteroventral margin smooth (Figure 29P– R). Remarks: In shape and opaque patch pattern, Ba. thyreoides resembles Ba. hypsiliformis. In side by side comparison of specimens, it differs by the shape of the central opaque spot and the less strongly etched punctation, so that the mid-lateral fields are almost smooth and more transparent than in Ba. hypsiliformis. Ba. thyreoides tends to be larger than Ba. hypsiliformis, although there is some overlap between the LV and RV clusters for both species (Graph 5). In Ba. thyreoides the posterodorsal corner is lower and less distinct than in Ba. hypsiliformis, and the posterodorsal margin slopes more continuously. Ba. thyreoides generally has only an oblong central spot and indistinct or no dorsal spots in the RV, whereas the Y-shaped opaque pattern is densely developed in both valves of Ba. hypsiliformis. In the samples studied, Ba. thyreoides occurs only in the Bahamas, and Ba. hypsiliformis only in Belize and Roatan (Table 1). On the preponderance of available evidence, they are considered here to be separate species. In Graph 5, the LV cluster is strongly offset (up and to the right) relative to the RV cluster, which is the normal valve asymmetry in Bairdiidae. Both LV and RV clusters suggest sexual dimorphism, with the larger individuals likely to be females. One A–1 instar is recognized.