218 SYNOPTIC COLLECTION. 



The shell of Crepidula (No. 445, C. nivea C. B. Adams), 

 is slightly coiled and the last whorl is large and flaring 

 like that of Haliotis. In the inside a projecting horizon- 

 tal wall covers the posterior half of the shell. No. 446 

 shows the peculiar habit these animals possess of growing 

 on top of one another. Sometimes one species of Cre- 

 pidula (C. plana Say) takes possession of the inner side 

 of a univalve shell, often Lunatia heros (No. 446). Five 

 or six different stages are seen in this specimen. The 

 young shells are round and smooth, as compared with the 

 adults, which are long, narrow, and extremely flat. Owing 

 probably to the hidden situation in which they live, they 

 are nearly colorless. 



The gills in the family Calyptraeidae, to which Crepi- 

 dula belongs, are much more like Pelecypod gills than are 

 those of most Gastropods. The lamellae are long and 

 much like filaments and are strengthened by chitinous 

 rods. 1 



Since the primitive or embryonic shell is uncoiled and 

 imperforate, and in shape like a cap or the beginning of 

 a cone, we know that the spiral condition is a secondary 

 one. It would seem most probable that before the perfect 

 spiral form was attained the cone would be loosely coiled, 

 arid that only after many generations and a long period 

 of time could we have the complete amalgamation of the 

 inner wall of a whorl to the preceding whorl which marks 

 the tightly coiled spirals of many existing species. Since, 

 also, the primitive shell is usually both smooth and color- 

 less or nearly so, it follows that the highly ornamented 

 and brilliantly colored shells are far removed from the 

 primitive form. The ontogeny of many Gastropods has 

 not been worked out, so that it is impossible to give a 

 classification which shall show the genetic relationships 

 of all the different species. It is most probable that the 

 ancestral form, represented by the embryonic shell of 



1 Dall, Bull. Mus. Comp. Zool., XVIII, 1889, p. 285. 



