BRACmOPODA. 193 



18S4. Orthis, Walcott. Palaeontology Eui-eka Dist., p. 74, pi. xi, tig. 4. 



1885. OH/iii, FoBBSTE. Bull. Denison Univ., vol. i, p. 82, pi. xiii, &g. 12. 



1889. Oitliis, Nettblroth. Kentucky Fossil Shells, p. 38, pi. xxxiv, fig-. 30. 



1890. Orthis, Foerste. Pioo. Boston Soc. N;it. Hist., vol. xxiv, p. 308, pi. vi, figs. 4, 5. 



The distinguishing features of these shells are the plano-convex contour ; the 

 strong, sharp and comparatively few costoo, rarely, if ever, bifurcating ; the 

 elevated and somewhat incurved cardinal area on the pedicle-valve ; the relatively 

 slight development of the dental hunellae, which do not extend the entire length 

 of the umbonal cavity. The cardinal process on the brachial valve is an elongate, 

 vertical plate, extending from the apex the whole length of the delthyrium, thus 

 longitudinally dividing the deep deltidial cavity. It is usually simple, both on 

 the outer edge and at its distal extremity. 



In this group of orthids, more frequently than elsewhere, we find a charac- 

 ter i-arely developed in any stage of growth, viz. : the existence of a transverse 

 apical plate in the delthyrium of the pedicle-valve. This is probably homologous 

 to the apical plate of the Spirifers, but is wholly distinct in origin from the 

 covering of the delthja-ium. The greatest development attained by this feat- 

 ure, in any of the numerous species of Orthis studied, is to be found in 0. 

 tricenaria of the Trenton and Hudson faunas ; it has also been observed in 0. 

 calligramma, var. Davidsoni, although it does not appear in any of the figures of 

 this species and its varieties given by Mr. Davidson, nor is any mention made 

 of it in his descriptions.* Its appearance in this genus, and especially in the 

 typical species of the genus, is interesting, but it can not be embraced in the 

 diagnostic characters since its presence appears to be largely of a specific 

 value, and the degree of its development dependent upon the stage of growth. 



The muscular scar of the pedicle-valve of 0. calligramma is a subelliptical 

 area scarcely longer than the cardinal face, faintly impressed, and its compo- 

 nents rarely distinguishable. 



The structure of the shell is compactly fibrous and impunctate. Specimens of 

 Orthis calligramma, var. Davidsoni, from Gotland, show openings of oblique tubules 

 on the external surface, always situated upon the keels of the costae. These 

 are sparse and irregularly scattered, but of similar nature to those seen in the 



* See British Silurian Brachiopoda, p. 240, pi. xxxv, figs. 1-19. 



