154 SYNOPSIS OF THE CHARACTERS OF THE 



Atrypa indentata. Sow. 



Atrypa indentata. Sow. Geol. Trans. 



Sp. Ch. — Transversely oval, convex; front inclented, beaks very small, prominent; abdominal edge of the 

 lower valve elevated, surface smooth. 



The surface of the upper valve is evenly convex, no mesial ridge being formed by the elevation of the 

 front margin ; the form is nearly orbicular, shghtly wider than long. Length three lines, width three and a 

 half lines. 



Atrypa insperata. Phil. 



Terebratula (Atrypa) insperata. Phil. Pal. Fos. 

 Sp. Ch. — Longitudinally oval, depressed; surface radiated with large, rounded, dichotomous ribs, about 

 six of them in the middle of the dorsal valve slightly elevated. 



A single very imperfect specimen of this species has occurred. 



Atrypa isorhyncha. M'Coy. (PI. XVIII. fig. 8). 



Sp. Ch. — Shell subglobose, depth exceeding the width, rather square, strongly plaited ; mesial elevation 

 square, of four plaits ; beaks obtuse, nearly equal, with a triangular flattened space on each side ; plaits twenty, 

 obtusely angular. 



The flattened or very slightly concave spaces on each side of the beak closely ally this remarkable fossil 

 with such species as the T. pUcatiUs and T. serruta of the Oolite, and the few other fossils which possess this 

 curious character. 



It may be known from the T. excavata of Phillips by the greater number of its plaits, which in that species 

 rarely exceed seven. Length ten lines, width one inch, depth one inch one line. 



Atrypa juvenis. Sow. 



Atrypa juvenis. Soic. Geol. Trans. — Terebratula juvenis. Phil. Pal. Fos. 



Sp, Ch. — Longitudinally ovate, narrow towards the front, depressed, smooth; beak of dorsal valve very 

 small, much incurved, strongly angulated at the sides. 



The outline of this species is very remarkable, from the greatest transverse diameter being near the beak. 

 Very rare. 



Atrypa lachryma. Sow. 



Atrypa lachryma. Sow. Geol. Trans. 



Sp. Ch. — Longitudinally ovate, valves nearly equal, very convex; beaks very small; front margin with a 

 small, deep sinus ; mesial sulcus of the dorsal valve broad, flat ; surface smooth. 



This little shell closely resembles the young of T. sacculus, but is obviously distinct, by its raised margin 

 and small beaks. Length five lines, width four lines, depth three and a half lines. 



Atrypa LATicLivA. M'Coy. (PI. XXII. fig. 16). 



Sp. Ch. — Transversely rhomboidal, length two-thirds the width, gibbous; middle of the shell elevated in 

 front, with three obtuse ribs, reaching nearly to the beak ; between the mesial elevation and the sides is a smooth 

 space, equal in breadth to the mesial elevation ; sides small, compressed, with three obtuse ridges, reaching half 

 way to the beak. 



This species is remarkable for the small number of its ribs, and the broad space between tliose of tJie me- 

 sial elevation and the sides. Length six lines, width nine lines. 



