'20-2 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



parts on either side. The mouth area faces downwards, 

 and is placed on the end of the freely-projecting truncated 

 end of the demi-cone, to which the term " snout'' has been 

 applied. The form of the mouth and lips is evidently 

 adapted to the process of scraping forwards by which the 

 animal feeds, and there is a marked contrast with the 

 vertically elongated seizing mouth of a Haliotis or a 

 Pleurotomaria. The snout is devoid of any power of intro- 

 version, and has no claim to be styled a " proboscis." 



An elongated cephalic tentacle, swollen at its base, is 

 attached on either side just behind the snout in line with 

 the front end of the retractor muscle band, which runs 

 back from the head to the foot. The eye is visible as a 

 small black pigment spot on the posterior side of the 

 thickened base of the tentacle. 



(c) The Mantle is a continuous flap or skirt, running 

 completely round the body and lining the marginal part 

 of the shell. The pallial impression is due to muscle 

 fibres which take origin not far from the mantle edge and 

 are inserted into the shell over a continuous band-like 

 area a little above the margin. The mantle is of greatest 

 extent in front, since here it roofs the nuchal cavity, the 

 opening of which has already been seen. The edge of 

 the mantle is pigmented and slightly thickened, and it 

 bears numerous pallial tentacles each arising from the 

 bottom of a pit into which it can be retracted. In a 

 living animal, observed when covered with sea water, 

 these organs can usually be seen projecting as a delicate 

 fringe beyond the edge of the shell. The pallial tentacles 

 are of two kinds, large and small, arranged in a regular 

 manner, the latter being by far the more numerous. 

 There may be over three hundred tentacles in a large 

 specimen, and the number appears to increase with age. 

 The mantle bounds externally a deep groove, the pallial 



