Bairdoppilata hypsiliformis Maddocks, 2025, sp. nov.

Bairdoppilata hypsiliformis sp. nov. (Graphs 5, 6; Figure 2K; 22A–L; 23A–R; 24A–U; 32O–Q; 33P–R) Derivation of Name: from ύψιλον, or upsilon, the 20 th letter of the Greek alphabet, plus the Latin suffix formis, -is, -e, having the shape of; in reference to the Y-shaped opaque patch pattern of the LV. Material Examined: 47 specimens. Types: Holotype specimen 4173Car, USNM 1751311. Paratype specimens 4092 RV, USNM 1751312; 4171 LV, USNM 1751313; illustrated specimens USNM 1751314–1751322. Type Locality: UH 2743. Belize, near Caye Chappel. 17 o 42’N, 88 o 03’W. 500–600 feet from shore, windward side of island, outside of barrier reef. Water depth 80 feet. Occurrence: Belize, Roatan (Table 1). Dimensions: Holotype specimen 4173Car, LV L 0.909 mm, LV H 0.567 mm, RV L 0.896 mm, RV H 0.495 mm. Paratype 4171 LV, LV L 0.831 mm, LV H 0.490 mm. Paratype 4092 RV, RV L 0.808 mm, RV H 0.421 mm. Paratype specimen 4189Car, L 0.816 mm, W 0.472 mm. Paratype specimen 4239 Car, L 0.815 mm, W 0.355 mm. See also Graph 5. LV are significantly longer and higher than RV. Presumed females are longer and have more posterodorsal expansion than males. Diagnosis: Carapace medium-sized in all proportions, broadly arched with indistinct posterodorsal angle; Yshaped opaque patch pattern, anterior and posterior opaque spots broad and vertically extended. Description: Carapace in right-lateral view broadly and asymmetrically arched, with substantial overreach, greatest height located slightly anterior to mid-length; with break in slope at indistinct posterodorsal angle; ventral margin approximately level or weakly convex; caudal process short, thick and bluntly terminated, only slightly swollen and not ridged (Figure 24D–H). Dorsal and ventral outlines slender, thickest at mid-length, sinuously tapering toward narrowly and nearly equally rounded ends (Figure 24P–T). Opaque patch pattern Y-shaped in LV, consisting of broad vertical streak, tapering to ventral margin, which joins two thick, divergent dorsal branches; fork between dorsal branches clear, cloudy, or dark (Figures 22A–F; 24A–C). RV with broad, continuous opaque streak, slightly thicker centrally, which extends from ventral margin to dorsal region. Anterior and posterior spots very broad, triangular, extended both dorsally and ventrally in both valves. Well-developed supplemental dentition. Valve edges broadly beveled or eroded (Figures 22G–J; 24K–O). ZC broad, with shallow to non-existent vestibules (Figure 22A–F). LV selvages curved, set close to margins; RV flange has broad nodular zone (Figures 22G–H; 24K, O). All margins of LV and RV smooth (Figures 22G–J; 24F, U). Surface of A–1 instar glossy, punctate, resembling adult (Figure 23A–F). LV opaque patch in A–1 instar a broad vertical streak of variable intensity and irregular contours, which extends from ventral to dorsal margin; dorsal spots separate or merged as dark streak; anterior and posterior spots thick and high, extended both vertically and horizontally (Figure 23A–B, I, K–R). Juvenile A–1 LV anteroventral margin with frill, posteroventral margin has spines or broken nubs; RV anteroventral margin with frill, posteroventral margin has fragmentary spines (Figure 23A–R). Remarks: Ba. hypsiliformis resembles Ba. thyreoides in lateral outline and opaque patch pattern, but most specimens show less transparency in the lateral fields. Ba. hypsiliformis tends to be smaller than Ba. thyreoides, although there is some overlap between the LV and RV clusters for both species (Graph 5). The posterodorsal region of Ba. hypsiliformis may be slightly expanded, producing a loaf-shaped or broad-shouldered lateral silhouette in supposed females, whereas in Ba. thyreoides the posterodorsal corner is lower and less distinct, and the dorsal margin slopes more steeply. The Y-shaped opaque pattern is densely developed in both valves of Ba. hypsiliformis, whereas Ba. thyreoides generally has only an oblong central spot and no dorsal spots in the RV. For Ba. hypsiliformis, the LV cluster is offset (up and to the right) with respect to the RV cluster, which is the usual relationship in Bairdiidae. Two non-overlapping size clusters are evident for both LV and RV in Graph 5, which may represent males (smaller) and females (larger). With some uncertainty, one juvenile instar is recognized. There are no significant differences in size or appearance between populations from Roatan Island (Honduras) and the Belize platform. Teeter (1975) did not illustrate this or any similar species in his survey of the Belize fauna.