DEVELOPMENr OF SOME SILURIAN BllACllIOPODA 333 



that the groove shows on the exterior, but just before matur- 

 ity it is either filled or introverted into the deltidial cavity. 



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

 Strophomenida', 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 descrip- 

 tions, a brief review of the hinge characters is here given, 

 showing more clearly their intimate relationships. 



In the three species, Leptcena rhomboidalis, Strophonella 

 striata, and Orthothetes subplanus, the initial form of the hinge 

 is the same. Each shows a slender callosity under the beak 

 of the dorsal valve, and a perforate pedicle-sheath in the 

 ventral valve, which does not entirely close the deltidial 

 opening. From this initial stage development proceeds in 

 a diiferent manner for each of the three species. Leptcena 

 rhomboidalis and Strophonella striata develop in a parallel 

 series until the individuals are about one-third grown in the 

 first species, and two-thirds full size in the second. That is, 

 the dorsal callosity and pedicle-sheath each increase uni- 

 formly in size up to these periods. Beyond this the diver- 

 gence is rapid and marked. In Leptcena rhomboidalis the 

 grooved callosity increases in size so as to nearly fill the 

 broad fissure in the ventral valve, while the pedicle-sheath 

 ceases growth, is atrophied and lost, although in many cases 

 the perforation persists. Strophonella striata continues its 

 hinge development without change, except that at full 

 maturity the groove on the callosity becomes introverted 

 into the pedicle-sheath. 



The third mode of development is exhibited by Orthothetes 

 subplanus, in which the pedicle -sheath does not increase 

 beyond its initial size, while the dorsal callosity develops up 

 to the maturity of the shell, and, as in Strophonella striata, 

 the groove is on the inner side. 



The function of this groove in the callosity of the dorsal 



