228 SYNOPTIC COLLECTION. 



shape with the flattened lips makes the shell more spe- 

 cialized than those of the other species of Cypraea already 

 described. Cypraea is a transitional form between those 

 shells that have the external spiral represented by the 

 young and those which have the spiral partly or wholly 

 internal, being covered by the growth of the later whorls. 



Ultimus gtbbosa Linn. (No. 492), does not show even 

 in youth any embryonic shell or spiral. The shell re- 

 volves so that each whorl covers the preceding. The 

 "margin of the aperture is thin and easily broken. In 

 course of growth the margin thickens and turns inward, 

 so that the adult has a strong incurving lip. 



A related species, Simnia acicularis Lam. (No. 493), 

 offers a striking example of mimicry. In the present 

 instance it has taken on the uniform deep purple color of 

 the coral on which it lives. When it chooses the yellow 

 coral, Rhipidogorgia, for its home, it assumes the yellow 

 color, and on white coral it is nearly white. Its foot is 

 narrow and well adapted for clinging to the coral. 



Transitional forms between marine Gastropods and 

 fresh-water or land species are found in Nerita, Neritina, 

 and the like. The transition from the one to the other 

 is governed by natural causes which we have already 

 found are not difficult to explain. 



The youngest portion of the shell of Nerita versicolor 

 Gmel. is yellow, smooth, and unmarked by color spots, 

 as seen in the first whorl of No. 494. In the next whorl 

 the surface is ridged, and the color is nearly white marked 

 by dark irregular spots. The teeth on the left of the 

 aperture are barely indicated and none are seen on the 

 right side. In (b) the ridges are more pronounced and 

 there are suggestions of teeth on the right side. In (c) 

 the youngest portion is colorless, as is the case with a 

 number of the older forms ; (e) has a predominance of 

 red color over the darker shade of the youngest shell, 

 and the teeth on both sides are well developed; (g) and 

 (h) illustrate the variation in color from a light to a dark 



