148 Annals of ihk Carnf.gie Museum. 



Spirifer and Delthyris are so exactly alike in their external form in 

 the youngest neanic stages that it is impossible to separate them. 



Strophomenidce. — The biconvex nepionic shells of Stropheodonta, 

 with a median dorsal fold and ventral sinus (which may or may not 

 be present) and the similar nepionic shells of CJwnetes indicate a pos- 

 sible common origin in some shell whose external form resembled 

 Tn'p/ccia, though probably not in that genus, which thus far has not 

 been found below the Calciferous, while Rafinesqitina, which would 

 seem to be the immediate ancestor of Siropheodonta, extends into the 

 Chazy and probably lower, without any marked change in form. The 

 early neanic stages oi Siropheodonta, before the appearance of the crenu- 

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



An interesting feature in the development of Siropheodonta 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 Spirifer, 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, though a 

 mucronate form in the. later neanic, and back to a rounded form in 

 the adult or senile condition. 'Jlie same thing occurs in the phylogeny 

 oi Spirifer, at least, for there are Niagara species with rounded cardi- 

 nal 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 Plaiystrophia. 



The difference between the varieties pectenacea and arctistriatiis of 

 Ortliotlictcs ihe/niini^e/isis is a good example of the effect of accelera- 

 tion in the development of certain characters. The steps in the 

 development of the two are exactly the same, but because the striae 

 are introduced at an earlier stage on one than on the other, the shells 

 differ greatly in appearance. 



ProdiictidcB. — The facts in the development of Ciwnetcs do not 

 seem to support the idea put forward by Hall and Clarke that Clionetes 

 might be descended from Plectanibouites, a shell which in many re- 

 spects much resembles Clionetes. The early neanic stages have an 

 outline which is much the same as that of Rafinesquina and Stro- 

 plieodonfa. The resemblance of the nepionic shell to Triplecia has 

 already been referred to, and this, with the other characters, relates 

 the shell to the Strophomenidai. It is clearly a transition form be- 

 tween the latter family and the Productidaj. 



