MOLLUSCA. 81 



like a slug (Limax) or naked snail ; and it has a small internal flat thin 

 shell, and four tentacles. 



Fam. 3. Unibr ell idee. This ffimily is monoicous; the branchire are 

 foliaceous, and disposed in a long line upon the right side. Umhrella has a 

 discoidal, external, dorsal shell, bearing some resemblance to that of a very 

 flat Patella ; a very large foot, and four tentacles. Some authors consider 

 this family as a distinct order under the name of Pomatobranchia. 



Oedek 7. Cyclobkanchia. In this order tlie branchiae, as the name 

 indicates, are arranged in a circle around the body above the foot. It 

 contains the two families Patellidm and Chitonidce., both of which are 

 monoicous, and have a long tongue armed with teeth. Blainville named 

 this order Cervicobranchia.^ from an erroneous opinion that the organs 

 generally received as branchiae are merely membranous plaits without the 

 function of branchiae, which he supposed should be looked for in a sao 

 above the neck. 



Fam. 1. Patellidm. The genus Patella {pi. 75, figs. 76, 77, 79) has a 

 simple conical shell with an oval or circular base ; inside with a submar- 

 ginal cicatrix open in front. The shell covers the animal like an inverted 

 cup. The animal is monoicous, creeping slowly upon a large foot, with 

 which it can adhere with great tenacity to rocks. Some species seem not 

 to move, except perhaps at long intervals, as they are found in depressions 

 which they have formed in the rocks, and which correspond to the outline 

 of the shell. 



Fam. 2. CJiitonidcB. Tlie shell of Chiton {pi. 76, fig. 55) differs so 

 much from that of Patella, that its position among the Mollusca was for a 

 lono; time doubtful. An examination of the animals of both o-enera at lensi;th 

 proved them to belong to allied families. The shell is oval, and divided 

 transversely into eight pieces resting upon the back of the animal, with the 

 lateral portions united by a coriaceous border. Thus constructed, the 

 Chitons have the power of bending themselves together. Tlie tentacles and 

 eyes are wanting, but the former are replaced by a veil. Their habits are 

 similar to those of Patella. In Chitonellus the animal is long and narrow, 

 and the border of the shell extends so far upon the back that the dorsal 

 plates are reduced to a small size. 



Order 8. Aspidobranchia (also incorrectly named Scutih^ancMa). In 

 this order the shell is allied to that of Patella in form, but it is emarginate, 

 or perforate. The order is divided into the two families Fissurellidm and 

 Calyj>trcBidcB. 



Fam. 1. Fissurellidm. The shell of Fissurella, with the perforation 

 through the apex, bears a distant resemblance to a key-hole scutcheon, and,, 

 excepting the perforation, resembles that of a Patella; but it is often too 

 small to cover the animal. The ])erforation admits water to the branchiae,, 

 and allows the faecal matter to be excluded. There are two pectinated 

 branchio3 in a cavity at the anterior part of the back ; and two tentacles,, 

 with the eyes at their external base. Fissurella grmca {pi. 75, fi^. 80). 

 Emarginida {fig. 82) difters from Fissurella in having the aperture 

 which admits water to the branchiae removed to the anterior margin of the 



285 



