Bairdoppilata sima, Maddocks, 2025

Bairdoppilata sima sp. nov.

(Graphs 2, 6; Figures 2L–M View FIGURE 2 ; 25A–T View FIGURE 25 ; 26A–L View FIGURE 26 ; 27A–V View FIGURE 27 ; 28A–S View FIGURE 28 ; 29A–I View FIGURE 29 ; 32W–X View FIGURE 32 ; 33Y–Z View FIGURE 33 )

Derivation of name: Latin adjective simus, -a, -um; meaning upturned, splayed; from the Greek σῑμός, meaning bent upwards. In architecture, the sima is the up-turned edge of a roof or eave, which functions as a gutter.

Material Examined: 36 specimens.

Types: Holotype specimen 4104Car, USNM 1751323 ; Paratype specimens 4150 RV, USNM 175124 ; 4165 LV, USNM 1751325 ; illustrated specimens USNM 1751326–1751330 .

Type Locality: UH 2638, Montego Bay , Jamaica. 18 o 28’N, 17 o 55’W. Sand from back-reef, water depth 20 feet GoogleMaps (6 m) GoogleMaps .

Occurrence: Jamaica, Roatan, U.S. Virgin Islands.

Dimensions: Holotype specimen 4104Car, LV L 0.846 mm, LV H 0.448 mm, RV L 846 , RV H 424 . Paratype specimen 4165 LV, L 0.816 mm, H 0.457 mm . Paratype specimen 4150 RV, L 0.891 mm, H 0.472 mm . Paratype specimen 4105 Car, L 0.859 mm, W 0.379 mm. See also Graph 2. In Graph 2, the LV cluster is relatively compact, but the RV cluster is more drawn out. Some RV may even be longer than LV, because of the extended anteroventral flange .

Diagnosis: Carapace short and low; edges of both valves subtly flared to form sima; lateral surface not abruptly compressed marginally; LV posteroventral region not prolonged, without spines; RV may reach or overreach LV anteriorly and posteroventrally, because of exaggerated width of flanges.

Description: Carapace broadly and nearly symmetrically arched in right-lateral view, highest anterior to mid-length, with moderate dorsal overreach; anterodorsal angle distinct, anteroventral margins unequally curved, not matching; RV anteroventral velum projecting beyond LV, posteroventral velum sagging below LV ( Figures 25A– B, M–N View FIGURE 25 ; 27A–M, P–Q View FIGURE 27 ; 29B, D, G View FIGURE 29 ). Caudal process prominent, slightly up-turned, thick, swollen but not ridged ( Figures 25A–B, D, E, G, J–K, M View FIGURE 25 ; 27A–I, K–L View FIGURE 27 ; 29C, I View FIGURE 29 ). Marginal region of both valves compressed, with edge flaring or subtly up-curved, forming shallow concavity or marginal channel (sima) ( Figure 27J–M View FIGURE 27 ). Dorsal and ventral outlines elongate, thickest near mid-length, with gently curved flanks, tapering smoothly to anterior end and more acutely produced posterior end; valve overreach reversed (RV>LV) in some specimens because of expanded RV flanges; edges not meeting because of flared margins (sima), nodular openings of RPC visible in gape ( Figure 27N–S View FIGURE 27 ).

Opaque patch pattern symmetrical in both valves, large, circular, diffuse, occupying much of central area, continuing dorsally as narrower stripe; no anterior or posterior spots ( Figure 25A–B, M–N View FIGURE 25 ). In juveniles, central stripe narrower, lighter ( Figure 26A–D View FIGURE 26 ).

Hinges with well-developed ligamentary striation; conspicuous supplemental dentition ( Figure 28A–B, E–O, Q–S View FIGURE 28 ). ZC wide, with vestibules of shallow to moderate depth ( Figures 25A–T View FIGURE 25 ; 28A–B, E–F View FIGURE 28 ). Anteroventral and posteroventral selvages sinuous to straight in both valves ( Figures 25C, F, H–I, L, M View FIGURE 25 ; 28A–B View FIGURE 28 ; 29A–B View FIGURE 29 ). Anterior and posteroventral vela very deep, especially in RV, with wide nodular zone at base of selvage ( Figures 25E–I, K–L, Q–R View FIGURE 25 ; 28G–H, Q View FIGURE 28 ; 29A–G, I View FIGURE 29 ).

Both LV and RV of the juvenile instar A–1 have smooth anteroventral and posteroventral margins ( Figure 26A–H View FIGURE 26 ).

Remarks: Ba. sima resembles Ba. fimbriata in general outline, carapace inflation, and densely punctate surface. Ideally, Ba. fimbriata is distinguished by the spinose posteroventral projection of the LV; whereas Ba. sima is distinguished by the flared edges and submarginal sima of both valves, and the conspicuously sagging posteroventral velum of the RV. RV of Ba. fimbriata and LV of Ba. sima are less remarkable, however, and isolated or abraded specimens are not as easy to assign.

In Graph 2, both the LV and RV clusters of Ba. sima are rather compact, and there is insufficient evidence to distinguish hypothetical sexual dimorphism. LV of Ba. sima are not as high in proportion to length as Ba. fimbriata , but RV of the two species show no clear distinction in H:L proportions. The single RV from Roatan and the U.S. Virgin Islands do not differ in size from the Jamaica population.

In dorsal and right-lateral views of Ba.sima there may be reversed overreach, both anteriorly and posteroventrally, because of the exaggerated RV vela; whereas Ba. fimbriata shows little if any departure from normal bairdian overreach (LV>RV). In Ba. sima the curvature of the lateral flanks is gradual, without abrupt marginal constriction; the valve edge is weakly recurved to form a shallow concavity (sima), and a gap may be evident between posteroventral valve edges in ventral view. In Ba. fimbriata the LV is more abruptly inflated centrally, with a surrounding, more compressed marginal brim.

Compared to Ba. fithianae , Ba. sima is not as high, relative to length. The caudal process of Ba. sima tends to be less inflated than in Ba. fithianae . The anteroventral and posteroventral flanges of the RV are of variable width in Ba. sima , from moderately to extremely wide, but they are of narrow to moderate width in Ba. fithianae .

Ba. sima differs from most species of Bairdoppilata by the exaggerated RV vela. A somewhat comparable but less extreme feature was illustrated for the RV of Ba. japonica by Horikoshi et al. (2019, p. 457, Figs. 4C–D View FIGURE 4 , 6B View FIGURE 6 , 7–9 View FIGURE 7 View FIGURE 8 View FIGURE 9 ), but it does not protrude beyond or hang below the margins of the LV. Ba. japonica has posteroventral marginal denticles in the LV and striate anteroventral and posteroventral frills in the RV, but Ba. sima lacks marginal denticles. Overall, Ba. japonica does not resemble Ba. sima , being non-caudate and much less elongate, and the valve margins are not flared to form a sima.