332 STUDIES IN EVOLUTION 



tion took place. The modifications in the convexity of the 

 valves is of more importance in this species, and can be read- 

 ily observed. In the young individuals, up to about one- 

 third full size, the ventral valve is slightly convex and the 

 dorsal valve nearly flat. Further growth of the shell changes 

 these relations, by the gradual deflection of the margin, until 

 the general form of the ventral valve is concave and the dorsal 

 valve is convex. 



The radii appear very early in the growth of the shell, the 

 smallest individual having eleven on the ventral valve, the 

 majority of which extend to the umbo. They probably first 

 appeared in pairs, and are found to increase in number after- 

 ward by simple intercalation. 



The hinge-area is developed upon both valves in all stages 

 of growth, although in the early stages the cardinal area of 

 the dorsal valve is very narrow, but gradually increases, until 

 at maturity it is nearly equal to the area of the ventral valve. 



The pedicle-tube is at first cylindrical and short. Advanc- 

 ing in the series, it is found to become conical from growth 

 and from the widening of the fissure, until in full-grown 

 specimens it is wider than high. A careful examination 

 reveals the perforation in all stages of the development of 

 the shell. It is, however, very minute, and it is not prob- 

 able that the extremely small peduncle could have performed 

 its full function. Indeed, it may be surmised that in none 

 of the three strophomenoid species here described was the 

 fleshy arm sufficiently strong in mature individuals to serve 

 as a secure support to the shell. In the embryonic forms it 

 was a more important organ. 



The hinge of the young shell illustrated in figure 4, Plate 

 XVII, shows an excessively elongate, cylindrical pedicle- 

 tube, of which more than one-half the length is projected 

 above the beak. It must be considered as a supra-calcifica- 

 tion about the peduncle, and apparently indicates a more 

 complete functional extension. 



The grooved dorsal callosity appears in the beginning of 

 the series, and gradually increases in size, and detrudes so 



