SILURIAN BRACHIOPODA 29 



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 

 uniformly in size up to these periods. Beyond this the divergence is rapid 

 and marked. In Strophomcna rhoniboidalis, 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 de- 

 velopment without a 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 Streptorhynchus 

 subplaniim, 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 valve in the 

 strophomenoids has not been satisfactorily determined, its existence having 

 sometimes been considered as evidence of the perforation of this valve.* 

 In all young shells, it is evident that the passage of the pedicle is not 

 through this groove in the dorsal callosity, but through the apex of the 

 ventral valve by means of the channel which has been here termed the 

 pedicle-tube or sheath. In growth-stages where there can be no ques- 

 tion of the functional activity of this sheath, the dorsal callosity is already 

 grooved or sinuate. It might be surmised that the purpose of the groove 

 was to avoid compressing the pedicle when the valves were open, and this 

 it may have been to some extent ; but the evidence furnished by both 

 recent and fossil species indicates that the valves of the articulate brachio- 

 pods could be opened only a very slight degree. The groove persists in 

 species after the true pedicle-perforation in the ventral valve is closed and 

 functionally useless. Its origin appears to be due to the organic deposition 

 about the bases of the two interior cardinal processes, the interstitial area 

 of .slower deposition being represented by a fissure, groove or sinus. 



•Eleventh Rept. State Geologist Indiana, pp. 288, 289. 1882. 



