156 SYNOPSIS OF THE CHARACTERS OF THE 



This species in general form and striation is analogous to tlie A. desquamata, but that species is dis- 

 tino-uished, as its name implies, by the absence of the concentric, scale-like striation of the present shell. 

 Length one inch two lines, width one inch. 



Atrypa proava. PJlU. sp. 



Terebratula proava. Phil. Geol. York. 



Sp. Ch. — Longitudinally obovate ; beak of the dorsal valve large, produced ; front broad, rounded ; mesial 

 fold square ; surface radiated with numerous, obtusely rounded ridges, which reach little more than half way to 

 the beak, leaving the rostral portions smooth. 



A few small, obscure specimens only have occurred of this curious species. Length six lines. 



Atrypa pugnus. Mart. sp. 



Anomites pugnus. Martin, Pet. Derb — Terebratula pugnus. Sow. Miu. Con. — Terebratula pugnus. Phil. Geol. York. 

 p. 3^ Sp. Ch. — Ovato-deltoidal, smooth ; dorsal valve evenly convex ; front much elevated, 



with four or live very short, obtuse plaits in the middle, and three or four on each 

 side. 



The most remarkable character of this species is the shortness of the marginal plaits, 

 which from some fancied resemblance to the knuckles of a clenched fist, have suggested the 

 specific name. The ventral valve is evenly convex and smooth. Length nine lines, width 

 one inch. Fig. 34 exhibits the brachial appendages in this species, they are remarkably 

 small, and similar in form to those of most of the plaited species. 



Atrypa radialis. Phil. sp. 



Terebratula radialis. Phil. Geol. York. 



Sp. Cli. — Suborbicular, beaks pointed, small; valves almost eqtially convex; no mesial fold; surface 

 radiated with fine, equal, rounded ridges. 



This pretty little species is rather rare ; length usually about three lines, width the same. 



Atrypa reniformis. Mart. sp. 



Anomites reniformis. Mart. Pet. Derb.— Terebratula reniformis. Sow. Min. Con.. — Terebratula reniformis. Phil. 



Geol. York. 



Sp. Ch. — Reniform; front margin raised so as to form a square sinus, from whicli about four obtusely 

 rounded plaits reach nearly to the beak ; sides smooth, tumid, without plaits. 



This species is remarkable for the tumid or inflated appearance of the sides, and this in botli the dorsal and 

 ventral valves ; so that at the sides, the dorsal valve presents the unusual appearance of hanging below the 

 margin ; this character will easily distinguish the present species from those nearly allied ; the mesial plaits 

 are obtusely rounded, but the notches tliey form in the margin are acute. Length one inch, width one inch 

 five lines, depth seven lines. 



Atrypa sacculus. Mart. sp. 



Anomites sacculus. Marl. Pet. Derb. — Terebratula saccidus. Sow. Min. Con. — Terebratula sacculus. Phil. Geol. York. 



Sp. Ch. — Obovate, very gibbous, edge rounded, obtuse; front indented, a deep, mesial furrow ; surface 

 smooth. 



This little shell is very gibbous ; it differs from all the varieties of the A. hastata in the great convexity 

 of its valves, blunt, rounded edges, the form of the indentation in the front of the deep mesial hollow, and its 

 small size, it rarely exceeding half an inch in length ; the mesial hollow in the dorsal valve is narrow, but so 



