124 SYNOPSIS OF THE CHARACTERS OF THE 



The obovate outline of this species distinguishes it from every other with which I am acquainted Length 

 one incli, width eleven lines. 



Orthis gibber a. Portk. sp. 



Atrypa gibbera. Portk. Geol. Eep. 



Sp. Ch. — (Young) semicircular, convex, front margin slightly raised ; (adult) semicylindrical, depth 

 exceeding the width ; sides subparallel, rostral portion of the shell flattened ; front of both valves deflected, 

 much produced, rounded ; margin of the dorsal valve deeply lobed at the sides, raised to a narrow, rounded 

 sinus in front ; cardinal area narrow ; surface very finely striated longitudinally, every fifth or sixth line thicker 

 than the intermediate ones. 



This curious species was figured on our plate nearly three years ago (previous to the publication of Captain 

 Portlock's Report) under the specific name alta, in allusion to the great height of the valves. I have, how- 

 ever, of course adopted Captain Portlock's specific name, as first published. The dorsal valve resembles a Bel- 

 hrophon bilobatus in sha][)e ; the striation resembles that of some varieties of the O . resupinata. Depth of 

 both valves one inch five lines, width one inch one line, length the same. 



Orthis granulosa. Phil. 



Orthis granulosa. P/iil. Pal. Fos. 



Sp. Ch. — Orbicular, flat; hinge-line two-thirds the width of the shell ; beaks very small, pointed; sur- 

 face strongly radiated, with angular, granulose ridges, which become doubled in number as they approach 

 the margin. 



A rare species, easily distinguished by its numerous, dichotomous, tuberculated striee. It resembles the 

 O. testudinaria and O. canalis of the Silurian rocks. Length three lines, width four lines. 



Orthis interlineata. Sow. 



Orthis interlineata. Sow. Geol. Trans, (pars.) — Orthis interlineata. Phil. Pal. Fos. 



»S};. Ch. — Transversely oval, depressed; beaks small, prominent; surface with strong, radiating striae, 

 increasing in number towards the margin ; cardinal teeth diverging, enclosing a large, cordiform space. 



This species is distinguished from the O. parallela by its greater width and large cordiform impressions; 

 from O. arcuata, by the directness and lesser width of its stria;. Length nine lines, width one inch three lines. 



Orthis Kellii. M'Coy. (PL XXII. fig. 4). 



Sp. Ch. — Truncato-orbicular, hinge-line slightly less than the width of the shell ; cardinal angles obtuse ; 

 ventral valve very convex, with a wide, rounded, mesial furrow ; dorsal valve shallow, slightly concave ; surface 

 of both valves radiated with very fine, close, unequal ridges and striae. 



I am indebted to Mr. John Kelly for directing my attention to this species ; it seems to be constantly dis- 

 tinguished from the O. crenistria, Phil., by its deep, rounded mesial fuiTow, which also indents the front margin, 

 so that it closely resembles in shape the O. bilobata, Sow.; it is also much more convex, and is particularly 

 gibbous near the beak, from which the surlace is regularly arched to the margin, while the O. crenistria is 

 flattened, or slightly concave near the beak, and is either nearly flat, or in old specimens suddenly rounded 

 to a short, deflected margin, as in the Z/. e«(?/y;jAfl!; the cardinal angles of the O. crenistria, Phil., are acute, 

 so that the width is nearly double the length ; while, on the other hand, in the O. Kellii, M'Coy, the cardinal 

 angles are obtuse, the hinge-line being rather less than the width of the shell, and the length is three-fourths 



