Argyrotheca barrettiana (Davidson, 1866)
Pl. 3, figs. 1, 2.
Material – One hundred and twenty shells of this species have been collected from localities on both Jamaica and Barbados.
Diagnosis – Medium to large, usually transverse Argyrotheca species; multicostellate with usually eight costae and costellae, increasing by intercalation up to 21 on the largest shell.
Description – Medium-sized, ventribiconvex valves of transverse to semicircular outline with maximum width either at or near hinge line or mid-valve length; cardinal extremities rounded, acute or perpendicular. Ventral valve about four-fifths as long as wide and about two-fifths as deep as long. Anterior profile with maximum convexity medianly where valve surface slightly carinate; flanks flat or weakly concave particularly near lateral margin. Lateral profile uniformly convex; umbo subdued. Ventral interarea flat to slightly curved, apsacline and about one-third valve length. Delthyrium large and open.
Dorsal valve over four-fifths as long as wide and about one-fourth as deep as long. Anterior profile with narrow sulcus originating near umbo; flanks slightly convex adjacent to sulcus, but flattened laterally. Dorsal interarea short and anacline. Ornament of strong costae and more rarely costellae; median sulcus with pair of costellae arising by internal branching within 5 mm growth stage. Four-21 ribs present on 4, 0, 1, 1,17, 3, 6, 1, 0, 2, 0, 1, 1, 3, 0, 0, 0 and 1 valves; median costae of average thickness 0.8 mm at 5 mm growth stage. Accentuated growth lamellae numbering about 3-6 per mm at 5 mm growth stage more marked on larger valves. Valve surfaces densely punctate.
Discussion – The outline and profile of this species together with the style of radial ornament are most similar to those of A. barrettiana (Davidson), a Recent species, described from the Caribbean (Harper, 1993). The large amount of material now available for study suggests that the Pleistocene form is, in fact, conspecific with Davidson’s living species. A sample of 25 specimens from Jamaica was compared, by PCA of the correlation matrix, with two specimens from Barbados that yielded a comparable set of complete measurements. The two specimens from Barbados are separated on the third eigenvector, responsible for 2.5 % of the sample variation. Both specimens have higher positive scores on this eigenvector than those of the Jamaican sample; maximum width (mw) has high negative loading on this eigenvector indicating that the Barbados specimens are relatively narrower than those from Jamaica. The Barbados material is currently insufficient to form the basis for the new subspecies, suggested by the multivariate analysis.
Although Cooper redescribed A. barrettiana (Davidson) from the Caribbean Sea, he did not provide a modern diagnosis of the species. That is provided here.
Occurrence – This species is common on Jamaica at the type section of the Manchioneal Formation (J2) and Christmas River (J3), and occurs at Cruikshank Bay (J1) and Folly Point (J5); approximately 100 shells of this species are now known from the Jamaican localities. On Barbados, the species is most common at Skeete’s Bay, Whitehaven (B1), but it also occurs at Spring Bay, Ragged Point (B3) and the Arawak Cement Quarry (B4) together yielding 20 shells.