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



anteriorly, but bounded on each side by the deltidial plates. Shell 

 non-punctate. 



Group 1. Plicate. 

 1. Plicos^. 



a. Pugnacece. T. acuta. 



T. acuminata, Soiv. T. bidens, Phil. 

 T. pugnus, Sow. (3. Concinnce. 



T. riugens. T. concinna, Sou\ 



T. varians, Sckloth. T. obsoleta, Soiv. 



T. tetrahedra, Soiv. T. decorata. 

 T. triplicata, Phil. 



2. Dichotomy. 3. Striata. 



T. subsimilis. T. reticularis. 



T. spinosa. (T. affinis, Sow.) 



T. senticosa. T. aspera. 



The above two lists contain the names of the species only that 

 have been examined, and they include most of the typical forms of 

 Terebratula. 



In adopting the subordinate divisions of Von Buch, it is necessary 

 to state, that in his memoir no allusion is made to the important re- 

 lation exhibited between the structure of the shell and certain cha- 

 racters afforded by the position of the foramen with regard to the 

 deltidial area, and it is probably from these relations having been 

 overlooked that the genus Atrypd has become the receptacle of 

 many species which are well-marked and belong to one or the other 

 of the above-proposed groups, as for example T. hastata, T. sacculus, 

 &c., which are all punctated species, associated with others, T. pug- 

 nus, T.pleurodon and allied forms, all of which are non-punctated. 



The characters of the groups above- described, especially the Hy- 

 pothyridae, are more easily recognised in the species belonging to 

 the oolitic than to the palaeozoic series, in consequence of the beak 

 of the dorsal valve being more produced in those of the former than 

 in those of the latter system ; still, a careful examination of the pa- 

 laeozoic species can generally detect the embracing deltidial area. 



With regard to the geological distribution of the two different 

 sections, it will be found that the species belonging to the Hypo- 

 thyridae predominated considerably during the palaeozoic aera, a few 

 species only of this period belonging to the other section. In the 

 Jurassic and cretaceous epochs the Epithyridae increased, and even 

 outnumbered those of the other section ; and the same group ap- 

 pears at the existing period to include (with two exceptions, and 

 those northern forms) all the hitherto-described species of Tere- 

 bratula. 



In conclusion, it must be admitted, that however easily the above 

 characters may be recognised, as forming a primary basis for the 

 arrangement of the numerous species of Terebratula, it will yet re- 

 quire a minute investigation into the form and position of the inter- 

 nal apophysary system, which doubtless is as various in the fossil as 

 in the recent species, before we can finally decide on the subordi- 



