270 MOLLUSCA. 



In the following genera, which are separated from the Patellae, 

 the shell is perfectly symmetrical, as is also the position of the heart 

 and branchiae. In the 



FissuEELLA, Lam., 

 We perceive a broad fleshy disk under the abdomen, as in the Patellse, a 

 conical shell placed on the middle of the back, but not always completely 

 covering' it, and perforated at its summit by a small orifice, which affords a 

 passage to the water required for respiration; this orifice penetrates into the 

 cavity of the branchiae, situated on the fore part of the back; a cavity other- 

 wise widely opened above the head. A branchial comb is symmetrically 

 arranged on each side; the eyes are on the external base of the conical ten- 

 tacula, and the sides of the foot are furnished with a range of filaments. 

 There are two other genera, Emarginula and ParmopJiorus. 



ORDER IX. 



CYCLOBRANCHIATA. 



The branchiae of the Cyclobranchiata resemble small lamelice, or 

 little pyramids forming a cordon more or less complete under the 

 borders of the mantle, very nearly as in the Inferobranchiata. 

 Their heart varies as to situation. But two genera of this order are 

 known, in both of which the shell never approaches in the least to 

 the turbinated form. 



Patella, Lin. 



The entire body covered with a shell, formed of a single piece, in the form 

 of a broad-based cone; a cordon of little branchial lamellse under tlie margin 

 of the mantle; the head is furnished with a thick and short snout, and two 

 pointed tentacula, on the external base of which are the eyes; the mouth is 

 fleshy, and contains a spiny tongue, which inclines backwards, and is re- 

 flected deeply in the interior of the body. 



Chiton, Lin. 

 A range of testaceous and symmetrical scales along the back of the mantle, 

 but not occupying its whole breadth; edges of the mantle coriaceous, and 

 furnished either with a naked skin or little scales, which give it the appeai-- 

 ance of shagreen, or with spines, hairs, or setaceous fasciculi. Under these 

 edges, on each side, is a range of lamellar, pyramidal branchiae; and before, 

 a membranous veil on the mouth supplies the want of tentacula. 



