MOLLUSC A OR UNIVALVES. 237 



cameos, the ground being always formed of the 



innermost layer of the three, which is also 



generally the most transparent.* 



102. Kjcinula. — Shell oval, generally co- 

 vered externally with spines ; mouth notched 



and narrow, strongly toothed on 



each lip, by which the aperture 



is contracted; spire very short; 



operculum horny. Animal, head 



large ; proboscis short ; two tenta- 



cula with eyes in the middle ; foot rather large. 



— 9 species, recent. 



These shells are usually small, and when 

 young have no traces of teeth : they are all 

 natives of foreign countries, and are found 

 principally in the Indian seas on coral reefs 

 and rocks. The Ricinula horrida is a remark- 

 able species, being whitish, and covered with 

 black pointed knobs ; the mouth is full of teeth, 

 and is tinged with a violet colour, 

 f 103. Purpura. — Shell oval, thick, either 

 smooth or tuberculated ; spire short ; mouth 

 large and notched ; left lip flattened ; operculum 



* Gray, Philosophical Trans. 



