484 Forty-fifth Report on the State Museum. 



poses of a hinge. In the articulates the teeth lie at, or just 

 beneath the cardinal margin, and at either side of the delthyrium. 

 They are short processes, free at their extremities, which are fre- 

 quently curved upward or backward into their sockets in the 

 opposite valve, thus locking the valves in such a manner that it 

 often becomes impossible to separate them without breaking the 

 teeth. Both teeth and sockets vary considerably in their develop- 

 ment in different genera, and in different species of the same 

 genus, but they are invariably two in number and are always 

 constructed on the same plan. 



Articulation is also aided by the cardinal process, an apophysis 

 situated at the center of the hinge of the brachial valve, and which 



in certain extinct genera, as Stro- 



PHEODONTA, DeRBYA, NuCLEOSPIRA, 



Productus, Stringocephalus, is of a 

 very considerable size. This process 

 was a surface of attachment of cer- 

 tain muscles, which, by their con- 

 traction, opened the valves, and 

 its size is due to the fact that tes- 

 t- «n D *-~~" w *™ awm ~~ taceous matter has been rapidly 



Fig. 69.— Rafinesqiuna alternata. Inte- I 



rior of the brachial vaive. secreted about the extremities of 



the muscular bands. It is in the later stages of growtli 

 that the cardinal process obtains notable size, and that 

 usually in species where the pedicle has become atrophied 

 and is no longer an obstacle to its growth. The pos- 

 terior surface of this process, which was the surface of mus- 

 cular attachment, is variously lobed, furrowed or striated. 

 (Plates 9-22.) 



The cardinal process is frequently connected with, and often 

 merged into, an elevated central hinge-plate. This plate is not 

 found in genera where the cardinal process attains its highest 

 development (Strophomena, Dekbya, Orthothetes, Triplegia : 

 Plates 1(>, 17 and 18), but, rather in those whose brachia are pro- 

 vided with calcified supports (Meristella, Retzia, Athyris, Tere- 

 bbatula, etc.), and while it is in some degree a surface of muscular 

 attachment, it also serves as a support to the bases of the braehia 

 (crura,). 



36 



