298 Raymond— Common Devonian Brachiopods. 



articulation is like that in Chonetes. JSTo deitidial plates are 

 developed, and the pedicle is probably functional throughout 

 life. The cardinal process is very large, and of a peculiar type, 

 quite different from that of any of the Strophomenidse. 



Spiriferidce. —The marked difference in shape and relative 

 convexity of valves of Cyrtina in the early neanic stage, from 

 Sjpirifer and Delthyris in the same stage, together with the geo- 

 logical range, would seem to indicate that, while both may be 

 derived from the same ancestral stock, Cyrtina is not a modi- 

 fied Sjpirifer. Delthyris, Sjpirifer, Amboccelia, and Cyrtina 

 all start out with an equivalve nepionic shell and a pedicle 

 opening shared by both valves. But with the first changes 

 in later nepionic and early neanic stages, when the fold and 

 sinus appear, Cyrtina and Ambocmlia become strongly in- 

 equi valve, while in Sjpirifer and Delthyris the valves retain 

 for a short time their equality of convexity. In other words, 

 the generic habit is assumed immediately after leaving the 

 form that is common to all the members of the superfamily, 

 and Cyrtina passes through no Spirifer-like stage. 



Sjpirifer and Delthyris are so exactly alike in their external 

 form in the youngest neanic stages that it is impossible to 

 separate them. 



Strophome?iidce. — The biconvex nepionic shells of Stro- 

 jpheodonta, with a median dorsal fold and ventral sinus (which 

 may or may not be present), and the similar nepionic shells of 

 Chonetes indicate a possible common origin in some shell of the 

 Triplecia type, though not perhaps in that genus which thus far 

 has not been found below the Calciferous, while Rqfinesquina, 

 which would seem to be the immediate ancestor of Stro- 

 pheodonta, extends into the Chazy and probably lower, with- 

 out any marked change in form. The early neanic stages of 

 Strojpheodonta, before the appearance of the crenulations on 

 the hinge margin, are very similar to the adult Rafinesquina. 



An interesting feature in the development of Strojpheodonta 

 is the marked mucronation of the cardinal extremities of the 

 adolescent specimens. This mucronation disappears to a greater 

 or less extent in the older stages. This same thing is noticed in 

 Sjpirifer, and there many of the adults retain the mucronate 

 forms, but they are only a phase in the life of the genus. In 

 the ontogeny the outline changes from rounded forms in the 

 nepionic and early neanic stages, through a mucronate form in 

 the later neanic, and back to a rounded form in the adult or 

 senile condition. The same things occur in the phylogeny of 

 Sjpirifer, at least, for there are Niagara species with rounded 

 cardinal extremities, then a great development of the mucronate 

 types in the Lower and Middle Devonian, and a return to the 

 rounded forms in the Carboniferous. A similar change is seen 

 in Platystrophia. 



