Lign 1. — Tebebsattjla elongata (Permian). 

 Interior of dorsal valve, with part of the 



ventral one. 

 H, Rostral or dental plates of ventral valve. 

 S, Sockets of dorsal valve. 

 c, b. Hinge-plates. 

 L, Loop. 



A, Quadruple impression of the adductor 

 muscle. 



Lign. 2.— Tebebbatula vithea (recent). 

 Literior of dorsal valve. 

 d, Cardiaal process. 

 b. Hinge-plate, 

 s, Dental sockets. 

 L, Loop. 



A, Quadruple impression of the adductor, or 

 anterior and posterior occlusor muscle. 



The family Terebeatulid^ comprises many genera and snb-genera ; but 

 these do not all appear to possess an equal value or importance, and time alone 

 will decide how far we are justified in certain of the divisions that have been 

 proposed. Professors M'Coy and King are of opinion that Palaeozoic Tere- 

 iDratulse such as T. kastata^ T. sacculus, and T. vesicularis, should be separated 

 from Terebratula proper (such as T. vitrea, T. carnea, and T. bipUcata ) on 

 account of certain peculiarities, or differences, and have respectively proposed 

 Semimila and 'ExMhyris, Phillips, as generic denominations for their reception. 



The differences between Terebratula proper and the palseozoic species above- 

 mentioned are chiefly confined to the presence of prominent dental or rostral 

 plates in the one, and their almost total absence in the other, as well as in certain 

 details connected with the hinge-plate. On the otlier hand, the exterior charac- 

 ters are similar ; and in the interior, the loop offers the same dispositions — is 

 short and simply attached, the longitudinal branches being united by a trans- 

 versal band more or less bent up m the middle, as is seen in Lign. 2. The 

 muscular impressions are also similar, as well as the intimate sheU-stmctui^e. 

 Therefore, while provisionally locating the Carboniferous species under Tere- 

 bratula, it wiU be necessary to bear in mind those small differences observable 

 in the rostral cavity of the beak and hinge-plate which appeal- to distinguish 

 the Palaeozoic from the Mesozoic species. 



I. — Terebratula hastata. Sowerby. PL xii., figs. 1, 2. 



Terebratula hastata, J. de C. Sowerby, Min. Con. Tab. 446, fig. 1-3, 1824, and 

 Dav. Mon. Carb.,'p. 11, pi. i., figs. 1-12. 



The shell is usually elongated, oval, or obscui'ely pentagonal, rounded or 

 ti-uncated m front, and tapering at the back. The valves are convex, and 



