MOLLUSCA. 81 



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

 shell, and four tentacles. 



Fam. 3. TJunhrellidoe. This family is monoicous; the branchia3 are 

 foliaeeous, 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. 



Okder Y. Cyclobranchia. In this order the branchial, as the name 

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

 contains the two families Patdlulm and Chitonidm^ both of which are 

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

 this order Cermcoh'anchia^ from an erroneous opinion that the organs 

 generally received as branchife are merely membranous plaits without the 

 function of branchia?, which he supposed should be looked for in a sac 

 above the neck. 



Fann. 1. Patellidce. The genus Patella {j)!. 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. ChitonidcB. The shell of Chiton {j^l. 76, fig. 55) differs so 

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

 long time doubtful. An examination of the animals of both genera at length 

 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. The tentacles and 

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

 similar to those of Patella. In Chitonelhis 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 Scutihranchia). 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 Fisswellida?, and 

 Calyptroeid(B. 



Fam. 1. Fissurellidw. 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 perforation admits water to the branchiae, 

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

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

 with the eyes at their external base. Fissurella gneca {j)l. 75, fig. 80). 

 Fmargiimla {fi^. 82) differs from Fissurella in having the aperture 

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



385 



