135 



Okthis porcata. (McCoy.) 



Orthis porcata (McCoy). Silurian Fossils of Ireland, p. 32, PI. 3, Fig. 14, 1846. Also 

 in the British Palaozoic Fossils, p. 223, PI. 1, h, Figs. 41, 42, 1851-55. 



Orthis inplata (Salter). Memoirs of the Geol. Surv. of Great Britain, Vol. 2, p. 372, 

 PL 27, Fig. 3, 1858. 



Fig. 111. 

 Fig. 111. — Orthis Porcata. a, ventral, and b, side view. 



Description. — Shell varying from a medium to a large size ; semi- 

 elliptical or subquadrate ; hinge-line equal to or a little less than the 

 greatest width ; cardinal angles rounded or rectangular ; sides gently 

 convex or straight and subparallel ; all the front half of the shell miiformly 

 rounded, sometimes a portion in the middle of the front margin nearly 

 straight. Ventral valve somewhat flat ; the beak strongly elevated, usu- 

 ally a wide shallow concavity extending from the umbo to the front mar- 

 gin, where it is often nearly as wide as the whole shell ; a flat, gently 

 convex or gently concave slope from the umbo to the cardinal angles. In 

 some specimens, the middle portion of the shell, instead of being gently 

 concave, is either flat or gently convex. The most elevated point is usu- 

 ally from half a line to one line from the beak ; the latter small, scarcely 

 distinct from the cardinal edge and depressed a little below the umbo. 

 The area is large, and very nearly equal to the Avidth of the shell in those 

 specimens which have the cardinal angles rectangular, and somewhat 

 shorter when they are rounded ; it is also either flat or gently concave, 

 and the upper part slightly overhangs the hinge-hne. The foramen is 

 triangular, open to the beak, its width at the base about equal to the 

 height. Dorsal valve uniformly convex, varying greatly in the amount of 

 its gibbosity, the cardinal angles flattened and usually a little reflected ; 

 greatest elevation a little above the middle ; umbo obtusely rounded ; 

 beak small, scarcely distinct from the cardinal edge ; area one half the 

 size of that of the ventral valve, with a wide triangular foramen open to 

 the beak, and having in the middle a strong divaricator process : it is 

 either flat or gently concave, and sUghtly overhangs the hinge-line. 



