MOLLUSCA. 81 
like a slug (Limax) or naked snail; and it has a small internal flat thin 
shell, and four tentacles. 
Fam. 3. Umbrellide. This family is monoicous; the branchie are 
foliaceous, and disposed in a long line upon the right side. Umbrella 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. 
Orper 7. Cycroprancuta. In this order the branchize, as the name 
indicates, are arranged in a circle around the body above the foot. It 
contains the two families Patellide and Chitonide, 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 branchiz are merely membranous plaits without the 
function of branchiz, which he supposed should be looked for in a sac 
above the neck. 
Fam. 1. Patellide. The genus Patella (pl. 75, jigs. 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. Chitonide. The shell of Chiton (pl. 76, jig. 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 Chztonellus 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. 
Orver 8. AsprpoBRANcHIA (also incorrectly named Scutibranchia). 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 Aamulies Fissurellide and 
Calyptreide. 
Fam. 1. Fissurellide. The shell of Fissurella, with the paladin 
through the apex, bears a distant resemblance to a ieay she 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 branchie, 
and allows the fecal matter to be excluded. There are two pectinated 
branchize in a cavity at the anterior part of the back; and two tentacles, 
with the eyes at their external base. L%sswrella greca (pl. 75, jig. 80). 
Emarginula (fig. 82) differs from Fissurella in having the aperture 
which admits water to the branchie removed to the anterior margin of the 
285 
