386 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 22, 



gatcR there are two distinct forms ; one ( T. carnea, Sow.) in which 

 the foramen is small and the deltidium scarcely visible, while in the 

 other ( T. ovalis, Lam., T. longirostris, Nilss.) the foramen is large 

 and the deltidium well-developed. The Carinatce may be also di- 

 vided into (1) Sinuatce (T.perovalis, Sow.), having a large foramen 

 almost entirely formed by the dorsal valve, the deltidium bounding 

 but slightly the anterior portion, and (2) the AcutcB (T. resupinata, 

 Sow.), wnth an incurved beak, a small entire foramen, and the del- 

 tidium expanded at the base. 



In the second group, Hypothyridce, the beak is always acute and 

 never truncate ; the foramen is entirely below the beak and placed 

 completely within the deltidial area; the deltidium either nearly sur- 

 rounds the foramen, as in T. vespertilio, &c. and those species in 

 which the foramen is entire (see fig. 3), or it partly lines the sides 

 of the deltidial area in T. psittacea and those species where the fo- 

 ramen is not entire, but open anteriorly and bounded only by the 

 umbo of the ventral valve (fig. 4); the margin of the foramen, when 

 entire, is either simple, or thickened and produced by the expansion 

 outwards of the deltidium itself (see fig. 3 b). In this section all the 

 species have ^non- punctated structure., are mostly deeply plicated or 

 costated, generally wider than long, and they have usually an elevated 

 central portion representing the mesial ridge among the Spiriferae. 



This group, which is peculiarly marked by the absence of the 

 truncated beak and by the foramen or opening being always within 

 the deltidial area, includes only two recent species, and nearly all 

 the fossil ones arranged by Von Buch under the PugnacecB and Con- 

 cinncB, two minor divisions well-distinguished from each other; the 

 PugnacecB (T, acuminata, Sow.) having the margin of the ventral 

 valve more elevated than the middle, the beak incurved or some- 

 what adpressed against the ventral valve, and the foramen small and 

 entire, although the deltidium is not much developed ; the Con- 

 cinncB ( T. concinna, Sow. 6:c.) having the middle of the ventral 

 valve more elevated than the margin, the beak acute and produced, 

 the deltidium surrounding the foramen, and the margin in some 

 species ( T. vespertilio, T. depressa, &c.) thickened and produced 

 outwards. In other species of this group howevev {T. psittacea) the 

 foramen is not entire, the deltidium being only slightly developed 

 at the sides of the area. 



Founded on the above characters, and using the previous classifi- 

 cation of Von Buch and Professor Phillips for secondary divisions, 

 the genus Terebratula may be either divided into two sections of 

 Epithyridae and Hypothyridce, as stated above, or the second group 

 may become a subgenus, for which either the term Hypothyris, 

 Phillips, or Cyclothyris, as proposed by M'Coy, may be used. The 

 description of the genus therefore stands thus : — 



Terebratula, Llwyd. 



Shell inequivalved, dorsal valve perforated posteriorly; beak acute 

 or truncate, produced or recurved; perforation either above or 



