136 SYNOPSIS OF THE CHARACTERS OF THE 



Spieifera Ueii. Flem. 



Spirifera Urii. Flem. Brit. Anim — Atrypa unguiculus. Soic. Geol. Trans. — Spirifera unguiculus. Phil. Pal. Fos. 



Sp. Ch. — Hcmlspliei-ical ; dorsal valve very gibbous; beak large, pointed; ventral valve nearly flat; sur- 

 face smooth ; botli valves with a narrow, linear, mesial sulcus. 



This curious little shell is subject to much variation in its characters, but is at all times easily recognized 

 by its peculiar form, and the deep, narrow sulcus in both valves. 



Cyrtia. Dabnan. 



Gen. Ch. — Cardinal area very wide, triangular; beak of the dorsal valve pointed, not incurved; mesial 

 fold usually smooth. 



This group is admissible only as a subgenus of Spirifera, from which it is distinguished by its cardinal area 

 being triangular, and the beak of the dorsal valve very slightly incurved, characters, however, which vary very 

 much even in the same species. 



Cyrtia cuspidata. Mart. sp. 



Anomites cuspidatus. Martin, Pet. Derb. — Spirifer cuspidatus. Sow. Min. Con — Spirifera cuspidata. Phil. Geol. York. 



Sp. Ch. — Pyramidal ; cardinal area very large, flat, triangular, forming, in old specimens, an acute angle at 

 the beak of the dorsal valve ; mesial ridge and hollow broad, rounded, smooth ; fourteen rounded, entire ribs 

 on each side the mesial fold. 



This very well known shell was first noticed by Mr. Martin (in the Linnsean Transactions, vol. iv. p. 4), 

 who considered it to form a distinct division of the group now called Spirifers. It is placed by Professor Phil- 

 lips at the head of his division Cuspidata:, which is nearly equivalent to Dalman's genus, Cyrtia, which I have 

 adopted, considering it, however, as only a subgenus of Spirifera, its characters being obviously subordi- 

 nate to those of that genus, and of less importance than any of generic distinction in this family. The cardinal 

 area of the present shell varies with the age of the animal : in young individuals its height is less than the width 

 of the hinge-line ; in old it considerably exceeds it. The mesial fold is usually broad, rounded, and smooth, 

 but specimens are not rare in which the mesial elevation is flattened, and divided in the centre by a sulcus, as 

 in Cyrtia distans ; and I have also seen specimens in which the mesial hollow was obscurely ribbed, as in that 

 species. Length two inches, width the same. 



Cyrtia distans. Sow. sp. 



Spirifer distans. Sow. Min. Con. 



Sp. Ch. — Rhomboidal, length two-thirds the width, depth equal to the length of the smaller valve; about 

 fourteen rounded, entire ribs on each side the mesial ridge, which is wide, smooth, and divided by a sulcus in 

 the middle ; the corresponding hollow on the dorsal valve contains about eight longitudinal ribs. 



The ribs in the mesial hollow of this species are somewhat smaller than those on the sides ; the mesial ele- 

 vation is without plaits ; the cardinal area very wide, curved ; foramen small. Von Buch considers this species 

 as identical with the Spirifer speciosus inicropterus of Goldfuss. It differs, however, in having ribs in the 

 mesial hollow, its more numerous plaits, and the greater space between the beaks. Length one and a quarter 

 inches. 



Cyrtia dorsata. M'Coy. (PI. XXII. fig. 14). 



Sp. Ch. — Subrhomboidal, nearly twice as wide as long; dorsal and ventral valves evenly convex; beak 

 of dorsal valve large, straight ; cardinal area very large, triangular, slightly concave ; mesial fold indistinct, or 

 none ; surface coarsely and regularly striated longitudinally. 



This remarkable species has nearly the form of the Strigocephalus dorsatus, Gold., but is a true Cyrtia ; 



