134 Fossils of the Niagara and Clinton Groups, 



Genus Atrypa, (Dalman.) 



The shells of ttie Genus Atrypa are often of a globular form, but some- 

 times elongated or sub-triangular, and most pointed at the beaks, which are 

 small and incurved, or hook-shaped. The surface is sometimes smooth, but 

 often ornamented with a number of ridges which radiate fi-om the beak, (see 

 figures 11 and 13.) The dorsal valve is the shortest, and the beak of the 

 ventral usually curves over it. It has moreover an elevation or mesial fold 

 in many species extending from the beak to the base, while the ventral valve 

 has a corresponding depression or sinus. Within, the arms are coiled, form- 

 ing two conical spires, the bases of which are towards the ventral and the 

 spires in the hollow of the dorsal vabe. In the interior of the dorsal valve 

 the muscular impressions are separated by a small ridge extending from the 

 beak downward, and in the ventral valve they are situated in a saucer-shaped 

 depression under the beak. The beak is sometimes perforated by a small 

 circular aperture. 



This genus ranges from the Lower Silurian to the Devonian. Atrypa 

 is from the Greek, (a.) without ; and {trupa.) a perforation. Some of the 

 s])ecie3 are however perforated. 



Genus Spirifer, (Sowerby.) 



In this genus the ends of the spires are directed outwards towards the 

 angles of the shell instead of into the hollow of the dorsal valve, as in Atrypa. 

 These spires were first discovered in species of this genus, and hence the 

 name from the Latin, (Spira,) a spire ; and few, I bear. 



Tbe spirifers have usually a long straight hinge line, a mesial fold on 

 the dorsal, and a sinus in the ventral valve. They are either smooth or 

 ornamented with radiating ridges. The angles at the ends of the hinge line 

 are often extended, forming acute or rounded points or ears, as they are 

 sometimes called. The beaks are either straight or curved. Both valves 

 have an area often very small on the dorsal. Both have also a foramen, 

 partly closed in the ventral valve. 



The genus commences in the Lower Silm-ian and becomes extinct in the 

 Frias. 



The above are the principal characters of the five genera of Brachiopoda 

 most abundant in species in the Silurian rocks. As the interiors of the shells 

 are not often seen, we have thought it not necessary to incumber the reader 

 with more lengthened descriptions. By comparing the figures of the species 

 and collecting and examining specimens, the destinctions may be soon per- 

 ceived. It should be remarked that the species described in this work, as 

 belonging to the genus AtryjDa, have not been yet proved to belong 

 to that genus. Their internal characters are for the greater part unknown, 

 and they have therefore been all classified by Professor Hall as Atrypie 

 for the jDresent. 



The following are the species from the Clinton and Niagara Groups, 

 figured on plate 2 : — 



Fig. 1, {Orthis circuhis,) Clinton Group. — This little shell is nearly 



