BRACHIOPODA. 
193 
1884. Orthis, Walcott. Palaeontology Eureka Dist., p. 74, pi. xi, fig. 4. 
1885. Orthis, Foerste. Bull. Denison Univ., vol. i, p. 82, pi. xiii, fig. 12. 
1889. Orthis, Nettelroth. Kentucky Fossil Shells, p. 38, pi. xxxiv, fig. 30. 
1890. Orthis, Foerste. Proc. Boston Soc. Nat. 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 costae, rarely, if ever, bifurcating; the 
elevated and somewhat incurved cardinal area on the pedicle-valve; the relatively 
slight development of the dental lamellae, 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 rarely 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 delthyrium. 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. 
