belonging to the Class Palliobranchiata. 35 



that Terebratula hastata possesses " two flat triangular laminae*." 

 Finding the loop in the existing Terebratulas in the Permian 

 T. elongata goes far to prove that it is general in the genus. I 

 have also seen it in the Jurassic Terebratula trilineata collected 

 in Glaizedale, Yorkshire. 



As might be expected, there are several species which it is dif- 

 ficult to place in their true genus ; but where so many characters 

 are available, some, or one at least, may be found to assist us. 

 Hypothyris decussata{T. coarctata, Sow.), in consequence of having 

 the " beak truncate, perforate," one would be disposed to place it 

 in Terebratula ; but as Dr. Carpenter has ascertained that its shell- 

 tissue does not exhibit any perforations, it has on that account 

 been placed in Hypothyris. Terebratula truncata has a subapical 

 foramen, and therefore might be included in Hypothyris, but this 

 is strongly opposed by its possessing a loop and by the texture 

 of its shell. Hypothyris psittacea, from the absence of decided 

 plications, would not have been thus generically designated, but 

 for its subapical foramen and imperforate shell- tissue f. Hypo- 

 thyris Meyendorfi appears to have a truncate, perforate beak, but 

 its deep frontal sinus and its indisputable affinity (another point 

 not to be overlooked) to H. acuminatus and H. pugnus, in which 

 the foramen is subapical, are clearly in favour of the generic al- 

 location adopted in the synoptical table. A few more species re- 

 main to be noticed, viz. Terebratula oblong a, T. orbicularis and the 

 so-called T. rostrata : all of them have the plicated character of 

 Hypothyris joined to the form usual to Terebratula ; but the api- 

 cal foramen of T. oblonga and T. orbicularis proves that they are 

 true Terebratulas, while the subapical position of the foramen in 

 T. rostrata shows that it belongs to Hypothyris : in the case of 

 two of these species this generic allocation is completely confirmed 

 by the researches of Dr. Carpenter, who has ascertained that T. 

 oblonga possesses a perforate and T. rostrata an imperforate shell- 

 tissue. 



In speaking of the internal structure of Terebratula and Hy- 

 pothyris, I have refrained from alluding to the two condyle plates 

 to be seen in the rostral cavity of certain species, for this reason, 

 that they are found in both genera, though not so frequently in 



* Synopsis of the Carboniferous Fossils of Ireland, p. 153. 



t Hypothyris psittacea has occasionally been suggested to belong to 

 Atrypa : by restricting this genus to the spiral-bearing shells included in it in 

 the synopsis, Hypothyris psittacea will necessarily be excluded. The dif- 

 ference between the mollusk of the one and that of the other appears to have 

 been considerable, since in Atrypa the labial appendages were in all pro- 

 bability completely attached to spiral supports and therefore immobile ; but 

 in //. psittacea, notwithstanding their spiral form, they undoubtedly possess 

 considerable motion. [Vide Professor Owen's Memoir on the Anatomy of 

 the Brachiopoda, Zoological Transactions, vol. i. p. 150.] 



D2 



