67S 



MOLLUSCOIDEA. 



cavity, are folded upon themselves, and are supported by a cal- 

 careous loop ; and the mantle is strengthened by a copious develop- 

 ment of calcareous spicula. The inner layer of the shell is fibrous, 

 but the outer layer is tubulated. The geological range of the family 

 is from the Carboniferous to the present day. 



The type-genus of this family is Thecidium ( == Thecided) which 

 commences in the Upper Trias, is well represented in parts of the 

 Jurassic and Cretaceous rocks, and survives under two specific forms 

 at the present day. In this genus the shell (fig. 549) is plano- 

 convex, and is fixed by the substance of the beak of the ventral 

 valve, or may be free in the adult condition. The hinge-line is 

 straight, with powerful hinge-teeth in the ventral valve, this valve 

 being further distinguished by the presence of a large triangular 

 hinge-area, in which is a pseudo-deltidium. The margins of both 

 valves exhibit internally a broad granulated and thickened border. 

 In the dorsal valve is a prominent cardinal process, flanked by the 

 dental sockets, below which the brachial supports unite to form a 



slender bridge. The loop follows 

 the margin of the shell, generally 

 forming more or fewer lobes, and 

 is either confluent with the shell, 

 or is connected with a calcareous 

 network formed by the largely 

 developed spicula in the mantle. 

 Besides the Liassic types in- 

 cluded under the names of Eude- 

 se//a, Davidsonella, &n&Bactrynium, 

 this family includes the remarkable 

 Carboniferous types described by 

 Waagen under the names of Old- 

 hamina and Lyttonia. In these 

 singular forms the shell is of large 

 size, flat or gibbous, and attached 

 by the ventral valve. The hinge- 

 line is straight and short, without 

 an area or pseudo-deltidium. In- 

 ternally, the ventral valve exhibits 

 a median septum, flanked on both 

 sides by numerous, transverse and 

 oblique, lateral septal ridges. The dorsal valve is rudimentary, 

 "forming together with the brachial apparatus one strongly-lobed 

 shelly plate, which fits between the external septa of the large valve " 

 (Waagen). These abnormal types have hitherto been detected only 

 in the Carboniferous rocks of India, and they are regarded by 

 Waagen as forming a special sub-family of the Thecidiidce. 



Fig. 550. — a, Interior of the ventral valve 

 of ' Oldhamina decipiens, from the Carbonifer- 

 ous rocks of India ; b, Cardinal view of the 

 dorsal valve. (After Waagen.) 



