28 MEMOIRS OF THE STATE MUSEUM 



examination reveals the perforation in all stages of the development of the 

 shell. It is, however, very minute, and it is not probable that the extremely 

 small peduncle could have performed its full function. Indeed, it may 

 be surmised that in none of the three strophomenoid species here described 

 was the fleshy arm sufiiciently strong in mature individuals to serve as a 

 secure support to the shell. In the embryonic forms it was a more impor- 

 tant organ. 



The hinge of the young shell illustrated in figure 4, plate 3, shows 

 an excessively elongate, cylindrical pedicle-tube, of which more than one- 

 half the length is projected above the beak. It must be considered as a 

 supra-calcification about the peduncle, and apparently indicates a more 

 complete functional extension. 



The grooved dorsal callosity appears in the beginning of the series, and 

 gradually increases in size, and detrudes so that the groove shows on the 

 exterior, but just before maturity it is either filled, or introverted into 

 the deltidial cavity. 



The features of the hinge, fissure and callosity, in the Strophomenidae, 

 and their embryological development, seem to be peculiar to the group. 

 They are of special interest both on this account, and also because the 

 family has no living congeners. Although the separate characters have 

 been presented in detail in each of the preceding descriptions, a brief review 

 of the hinge characters is here given, showing more clearly their intimate 

 relationships. 



In the three species, Strophomcna rhomboidalis, Strophonella striata and 

 Streptorhynchus subplanum, the initial form of the hinge is the same. Each 

 shows a slender callosity under the beak of the dorsal valve, and a perfo- 

 rate pedicle-sheath in the ventral valve, which does not entirely close the 

 deltidial opening. From this initial stage, development proceeds in a dif- 

 ferent manner for each of the three species. Strophotneiia rhomboidalis and 

 Strophonella striata develop in a parallel series until the individuals are 



