318 STUDIES IN EVOLUTION 



readily distinguished from tlie latter by its flatter and shal- 

 lower dorsal valve and deeper ventral valve, features which 

 usually hold good for purposes of discrimination; but in 

 extremely early stages of growth the nearly equivalve form 

 of the shell makes the separation of the species very diffi- 

 cult, perhaps even impossible. Between the dimensions of 

 .5 X .75 mm. and 18.5 X 18 mm. (which is a little in excess 

 of average mature size) has been found every gradation in 

 size and development. The minute shell which serves as a 

 starting-point for the series may quite as well be taken as the 

 incipient shell of R. hybrida, as both its valves have the same 

 depth, while the cardinal areas and beaks show the same 

 character of development. As there can be no doubt of this 

 fact, it becomes impossible to determine whether a given 

 embryo, could it have grown to maturity, would have devel- 

 oped into R. hyhrida or D. elegantula. Until the embryos 

 reach a size of 2 or 2.5 mm. in length, their specific value is 

 undeterminable, and the specific individuality of D. elegantula 

 can be established only with the increasing depth of the ven- 

 tral valve from this point upward toward adolescence. 



Unless these observations are at fault (and they have been 

 made with great care), evidence here is very positive that 

 the diagnostic characters of species of this group may not be 

 assumed until the earlier stages of the existence of the shell 

 have passed. Indications of similar character are found 

 among the species of Camarotoeohia and Spirifer. The im- 

 portance of the fact is apparent and its significance will be 

 appreciated. 



Specific Characters. 



Mature Form (Plate XV, figures 10-12 a). — Oubline sub- 

 circular; hinge-line short, about one-half the width of the 

 shell, straight. 



Ventral valve elevated along the dorsum, which is arched 

 and slopes more rapidly toward the lateral than toward the 

 anterior margin ; greatest width below the hinge-line, about 

 half-way down the valve. Beak full, arched, incurved, and 



