334 STUDIES IN EVOLUTION 



valve in the strophomenoids has not been satisfactorily deter- 

 mined, 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 question of the functional activ- 

 ity 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 indi- 

 cates that the valves of the articulate brachiopods could be 

 opened only to 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. 



Mimulus waldronensis Miller and Dyer, 1878. 



(PLATE XVII, figures 9, 10.) 



Spirifera f waldronensis Miller and Dyer. Contributions to Palaeontology, 



Jour. Gin. Soc. Nat. Hist., April, 1878. 



Triplesia putillus Hall. Trans. Alb. Inst., vol. x, abstract, p. 16, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 298, pi. 27, 



figs. 19-22, 1882. 



This species is among the rarest of the Waldron Brachi- 

 opoda, and it is impossible to present a series representing the 

 variety and progress of development, as in some of the more 

 common forms. There were but two specimens, both adults, 

 discovered in the State Collection at the time of the publica- 

 tion of the " Descriptions of New Species of Fossils from the 



* Eleventh Rept. State Geologist Indiana, 288, 289, 1882. 



