390 STUDIES IN EVOLUTION 



never passed by Meristina rectirostris, which shows a uni- 

 form, open, triangular area in every period of growth. 



A further advanced state of progress initiates the deltidial 

 plates, which first appear as narrow laminae along the sides 

 of the area. The areal development of Spirifer crispus, 

 Camarotoechia neglecta, and C. acinus ceases at this point. 



In the next stadium the further growth of the deltidial 

 plates along their free edges gives them a triangular form, 

 and they tend to narrow the limits of the opening and define 

 the peduncular foramen. Spirifer niagarensis and Camaro- 

 toechia Whitii represent species which are arrested at this period. 



The completed growth shows the deltidial plates uniting 

 by symphysis along a median line, and enclosing near the 

 apex of the area a more or less circular pedicle perforation. 

 Rhynchotreta cuneata, Meristina Maria, Homoeospira evax, 

 etc., after passing through all the earlier conditions, reach 

 this limit of development. 



The results of senile and extravagant growth often oblit- 

 erate or degenerate the normal deltidial advancement, the 

 plates becoming thickened and their features obscured, while 

 in some species processes are given off, as in a number of the 

 Mesozoic Rhynchonellidse. 



The cardinal area of the Strophomenidse in its early phase 

 shows a small pedicle-sheath for the ventral valve and a 

 narrow grooved process under the beak of the dorsal. The 

 perforation for the passage of the peduncle does not mate- 

 rially increase in size with the growth of the shell and often 

 is obliterated, while the dorsal callosity usually reaches a 

 considerable development. 



Additional evidence of the degeneracy of the pedicle is 

 afforded by many species of other genera, which have a 

 calcareous attachment to foreign objects at the apex of the 

 ventral valve, the pedicle, therefore, becoming functionally 

 obsolete. 



Observations having some analogy with the facts here pre- 

 sented have been made, in a very restricted sense and usually 



