i 4 MEMOIRS OF THE STATE MUSEUM 



difficult, perhaps even impossible. Between the dimensions of . 5*. 75 mm. 

 and 18.5x18 mm. (which is a little in excess of average mature size), we 

 have 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 Orthis 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 developed into O. hybrida or O. elegantitla. Until the embryos 

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

 and the specific individuality of O. elegantula can be established only with 

 the increasing depth of the ventral valve from this point upward toward 

 adolescence. 



Unless our 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 genus 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 Rhynchonella and Spirifcr. The importance 

 of the fact is apparent and its significance will be appreciated. 



SPECIFIC CHARACTERS 



Mature Form (plate i, figs. 10-120). Outline subcircular ; 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 projecting over the cardinal area sufficiently to conceal 

 the foramen. Cardinal area broadly triangular, low, incurved ; foramen 

 triangular ; deltidial plates absent. 



Dorsal valve shallow, nearly flat, slightly rounded over the umbo, but 

 depressed toward the margins. A sharply defined sinus starts near the 



