40 CANADIAN FOSSILS. 



Specific character. — 0. rotundata pollicaris, valvd dorsali pland, ventrall 

 gibbd cardine, in latitudine testam (Bquantc ; ared magnd subcurvd ; 

 foramine angustissimo. Costce, radiantes convex^, circiter 30, interstitiis 

 angustissimis in valvd dorsali soepe Jiliferis ; striis transversis incon- 

 spicuis. 



Synonym. — O. tricenaria, Conrad, 1843. Proc. Acad. Nat. Hist. Soc, 

 vol. i., p. 333. Hall's Pal. New York, vol. i., p. 121, plate 32, 

 figure 8. 



There could scarcely be better examples of specific character, as 

 distinguished from mere variability, than are exhibited by four shells? 

 two American and two British, among the large plaited Orthides of 

 the Lower Silurian rocks. Of these, Orthis tricenaria, the subject of our 

 plate, is precisely analogous to the English form, O. actonite, Sowerby, 

 while 0. pectinella, of Conrad, another American species, with nume- 

 rous varieties, equally well represents the O. flabellidum of Snowdon. 

 Yet while all these species so much resemble one another that they 

 might easily be confounded, each is distinguished by characters as neat 

 as they are constant. O. Actonia, Siluria, 3d edition, p. 209, fig. 32, 

 has few strong angular plaits, frequently bi- or tri-furcate at their 

 ends, and the ventral or receiving valve is the convex one, the dorsal 

 being concave. In O.jlabellulum the reverse takes place, or the larger 

 ventral valve, although prominent at the beak, being much flattened 

 and actually concave, while the convex valve is the dorsal one. 

 This difference is constant, while the fewness of the ribs in both 

 distinguish them from the variable O. calligramma of Dalman, which 

 has both valves gently and equally convex. 



The same relative differences are observable in the two American 

 species above quoted. O. tricenaria has the ventral valve the large 

 one, and the other flat or even concave, while O. pectinella possesses 

 a well-defined convex dorsal valve and a concave ventral valve, the 

 beak of which is nevertheless prominent, as it is in O. jlabellulum. 

 Both again are distinct from the O. plicatella, which represents in 

 America the O. calligramma of Europe, and, like it, has both valves 

 convex. 



O. tricenaria is of a semi-oval or even subtrigonal form when 

 young, and in age is subquadrate, from the greater extension of the 

 front than the sides, with length and breadth very nearly equal. 

 Dorsal valve (fig 1), flat, concave at the beak, its hinge-line straight, 

 produced into very short ears ; ventral valve (fig. 2), strongly convex, 

 even gibbous, its beak much elevated, but not at all incurved ; 



