hedley: pterosoma a heteropod. 



335 



tail. The anterior extremity of the body is free from the lateral 

 expansion, and consists of a bell-shaped proboscis or buccal mass, 

 behind which are two small and slender tentacles ; behind these again, 

 just where the free portion of the head becomes attached, are the eyes. 

 In the centre of the back arises the crested fin^ whose muscular fibres 

 are interlaced diagonally. On the posterior edge of the fin a sucker is 

 discernible ; to the right anterior of the fin, in the male, the external 

 genitalia (Fig. 4) are situated. A median groove, in which the fin is 

 seated, extends to the tail, which latter tapers for some distance and 

 then continues as a slender thread, terminating without sucker or 

 other appendage. On the under surface, just beneath the fin, are 

 suspended the shell and its contents, from the anterior edge of which 

 protrude the branchice-like fingers (Fig. 2). The shell is carried with 

 its spiral end posteriorly; it is 4'5mm. long, 1-5 mm. broad and high, 

 apically consisting of a spiral, smooth, purple shell of 2^ whorls, at 

 right angles to which extends the adult boat-shaped shell, white, 

 glassy, shining, most bi'ittle, ornamented by a double lamellar keel 

 running from the embryonic shell along its periphery, from keel to 

 margin curved growth lines undulate the side of the shell. The 

 radula (Fig. 3) possesses the usual scythe-like lateral teeth of the 

 family, but the rachidian has an individuality distinct from that of 

 Carinaria or Atlanta, as figured {pp. cit., figs. 344, 346) by Fischer. 



\0L. I.— OCTOBER, 1895. 



22ff 



