GASTEROPODA PECTINIBRANCHIATA. 263 



nation nor syphon; the animal resembles the other Pectinibranchi- 

 ata. There is but one branchial comb transversely arranged on the 

 roof of the cavity, and its filaments are frequently very long. 



PILEOPSIS, Lam. 



A conical shell with a recurved and spiral summit, which has long 1 caused 

 it to be placed among- the Patellae; the branchiae are in one range under 

 the anterior margin of the branchial cavity; the proboscis is long-, and there 

 is a closely plaited membranous veil under the neck; the eyes are at the ex- 

 ternal base of the conical tentacula. The 



HIPPONYX, Defr. 



Would appear from the shell to be a fossil Pileopsis, very remarkable, how- 

 ever, for a bed, formed of calcareous matter, on which it rests, and which 

 probably exuded from the foot of the animal. 



CREPIDULA, Lam. 



The shell oval, with an obtuse horizontal point, directed obliquely backwards 

 and laterally, the aperture forming the base of the shell, which is half closed 

 beneath and behind by a horizontal plate. The abdominal sac which con- 

 tains the viscera is on this plate, the foot beneath, and the head and branchiae 

 forwards. The latter consists of a range of long- filaments attached under 

 the anterior margin of the branchial cavity. The eyes are at the external 

 base of two conical tentacula. The genus 



PILEOLUS, Sowerby, 



Appears to consist of Crepidulae in which the transverse plate occupies half 

 the aperture; their shell, however, is more like that of a Patella. They are 

 only found fossil. 



NAVICELLA, Lam. 



The shell resembles a Crepidula, except that the summitis symmetrical and 

 laid on the posterior margin, and that the horizontal plate is less salient. 

 The animal is also provided with an additional, irregularly shaped, testaceous 

 plate, horizontally connected with the superior surface of the muscular disk 

 of its foot, and covered by the abdominal sac, which it partially supports. 

 It is probably analogous to an operculum, but does not exercise its functions, 

 being in a measure situated internally. The animal has long tentacula, at 

 whose external base are pedicles which support the eyes. They inhabit 

 the rivers of hot countries. In the 



CALYPTR^EA, Lam. 



We observe a conical shell, in the hollow of which is a little lamina that 

 projects inwards, resembling the commencement of a columella, and that 



