CREPIDULA. 123 



Lesson, C. tomentosa (figs. 96-98), and C. maculata (fig. 99) of 

 Quoy. It is also C. Chinensis of Chenu, not Linn. 



Genus CREPIDULA, Lam., 1799. 



The slipper-limpets are quite as variable as either of the pre- 

 ceding groups, and the definitive arrangement of the species 

 will require continued observation of the growth of shells from 

 the same brood under diverse conditions. I take the species as 

 conventionally recognized, although fully aware of some facts 

 which, at least, militate strongly against the retention of some 

 of them. As in most attached genera the form and sculpture 

 are influenced by the surface of attachment ; thus a species flat 

 and suborbicular when attached to a similar surface will become 

 high and compressed if growing on the spire of a narrow- 

 whorled shell. 



C. unguiformis, Lam., is generally found on the inside of 

 large univalve shells, such as Naticas, Fulgurs, etc., and has the 

 upper surface concave, and the color white; and Gray thought 

 that the same species became 0. fornicata, Gmel., convex, with 

 radiating chestnut spots, when attached externally ; but ungui- 

 formis whilst appearing to prefer the interior station, yet is 

 sometimes found on the outside of shells, still preserving its 

 white color and long shape, but becoming somewhat convex. 



The admitted cosmopolitan distribution of C. unguiformis 

 points to a large geographical range in the other species ; yet 

 the protected station usually chosen by that species is probably 

 conducive to its spread and acclimation. 



Gray writes (Zool. Proc., 737, 1867): "Messrs. H. and A. 

 Adams formed the genus lanacus for the white flat specimens of 

 this genus. All the specimens of the different species that take 

 up their residence in the inside of the shell assume this form 

 and color. It has been suggested that, instead of being depend- 

 ent on the accident of locality, their living in such places is a 

 habit peculiar to the species, and a proof that they are of a dis- 

 tinct kind. This theory is set at rest by the fact that some 

 specimens of the shell show the two states ; that is to say, an 

 animal growing on the outside of a shell has moved to the inside, 

 and an animal that commenced life on the inside of a shell has 



