CHAP. IX. SOUTIBRANCHIA, OR LIMPETS. 243 



conic) is not oval, but forms a very narrow slit or cleft, 

 the position of which indicates the particular situation 

 of the branchial cavity. In the typical species, this slit 

 is at the base ; in the sub-genus Cemaria Leach, it is 

 in the apex ; while in Rimula it is central between the 

 apex and the margin. The most aberrant type is Par- 

 mophorus, where, as in all the tectibranchial types, the 

 animal is much larger than the shell, which thus be- 

 comes partly internal ; hence there is only a slight 

 emargination on the margin. Our new sub-genus He- 

 mitoma opens the passage between this and Emarginula ; 

 and thus the whole form a circle. The animal, like that 

 of Fissurella, has the margin of the foot fringed with 

 filaments, and the eyes pedunculated, but at the base of 

 the ten taenia. Cuvier says the mantle envelopes and 

 covers a great part of the shell ; and this is shown by 

 Riippell to be the case also in Parmophorus, but to a 

 greater extent. 



(226.) HippoNYx is one of the most remarkable of 

 the patelliform shells : it is cup-shaped, like Emar 

 ginuki, but has no fissure : it may be almost termed a 

 bivalve, since it forms a flat, thin, calcareous plate, which 

 covers that part of the rock on which the upper valve, 

 or true shell, reposes ; this latter so much resembles a 

 limpet, that it can only be known by its horseshoe 

 muscular impression : the common species are small, 

 and generally whitish. The animal figured by Mr. 

 Quoy has the mouth shaped like a short proboscis, to- 

 tally different from that of the Trochidfe, near to which 

 some authors approximate it ; the mantle is not fringed, 

 and the eyes and ten taenia are like those of Emarginula. 

 Several recent species from warm countries are now 

 known, and a few fossil ones. The sub-genus Cajmlis, 

 which has not this basal valve, seems to connect Hip- 

 ponyx with Emarginula, — which latter it resembles in 

 the cup-shaped form of its shell. The analogy of this 

 genus, in their shells, to the bivalve Branchiopoda, is 

 particularly remarkable. 



(227.) The genus Patella stands at the opposite 

 R 2 



