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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 aperture ; but in Atrypa, Spirifer, Or- 
this and Leptsena, 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. Yon 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 Leptmna 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 ( Gt/pidia 
Conchidium , Dalm.) is the most difficult to distinguish from 
Atrypa, and requires further consideration ; it, however, is 
perfectly distinct from Terebratula, with which Von Buch 
