12 



lines, with a thick tongue-like process between the two in 

 the larger valve. 



The small size of the area between the beaks, and the 

 short hinge-line, distinguish this genus from Spirifer, but 

 there is evidently a passage from one into the other ; there 

 can, however, be no difficulty in distinguishing the genus 

 Terebratula from both, if attention be paid to the struc- 

 ture of the beaks, the deltidium, the texture of the shell 

 (which in Terebratula is punctato-laminated, — a distinction 

 first pointed out by Mr. Morris), and the general habit. 

 The deltidium in the genus Terebratula is generally formed 

 of two pieces, and always opens at the extremity of the 

 beak, or (as Von Buch describes it) separates the muscle 

 of attachment from the hinge-line of the larger valve, 

 hence the circular apertqre ; but in Atrypa, Spirifer, Or- 

 this and Leptaena, it is composed of one immoveable piece 

 filling- up the beak, but sometimes leaving a sinus in the 

 middle of the hinge-line. In many species, especially of the 

 genera Atrypa and Spirifer, it is concave, and admits the 

 beak of the lesser valve to repose upon it ; in others it is 

 flat. Von Buch has mixed the species of the genus Atry- 

 pa with Terebratula ; he would have done better to have 

 united them with Delthyris, under which he includes Spi- 

 rifer ; for whenever there is an opening in the deltidium, it 

 is a sinus on the hinge-line, and not close to the apex of 

 the beak, as in Terebratula, which natural genus alone 

 comes under his definition. The flat area on each side the 

 deltidium is variable in size in all the genera which pos- 

 sess it: the species of Atrypa in which it is found have 

 it much less than the width of the shell, and bounded by 

 the curved surface of the beak. In Leptaena and Orthis it 

 is equal to the width of the shell, and occupies the whole 

 inner curve of the beak; a similar but smaller area oc- 

 curs in the other valve. The genus Pentamerus [Gi/pidia 

 Conchidimn, Dalm.) is the niost difficult to distinguish from 

 Atrypa, and requires further consideration ; it, however, is 

 perfectly distinct from Terebratula, with which Von Buch 



