9 



li(nitaii s]jecik,s of an iiitL'iiuediate layrr bet w ecu I he 

 middle and internal, but, unless this ronespoiids wilh 

 the inner and clearer portion (13, h) ispoken of above, we 

 have not found it. 



II. Soft Parts of Extkiuok. Before removal ol the 

 shell the prominent ventral foot, ill-dehned head, and 

 continuous mantle-skirt can be made out in ventral 

 (fig. o) and side view of the hod^^ 



[a] The oval muscular Foot has a well-defined wavy 

 edge which separates a smooth ventral surface from the 

 smooth sides. Laterally and posteriorly the latter rise 

 almost vertically to the insertion of the mantle, and 

 anteriorly become continuous w^itli the head. 



[h) The Head, as seen from the side, is a prominent 

 muscular projection overhanging the front end of the 

 foot (fig. 5). In shape it may be compared to a truncated 

 demi-cone with the cut end turned forwards, rounded off, 

 and bent somewhat downwards to form a snout, the 

 circular muscle in the wall of which is very well 

 developed, and is practically a sphincter. The fiat side of 

 the demi-cone faces downwards, and is marked off by a 

 distinct transverse groove from the snout. On either side 

 of the groove is a well-marked retractor muscle band con- 

 necting the head with the side of the foot. The upwardly 

 facing curved surface of the demi-cone is continued back- 

 wards till it passes into and becomes the fioor of a cavity 

 of which the wide opening can be seen above tbe head. 

 This is the Nuchal cavity. 



The transversely oval mouth situated at the fiont end 

 of the head is bounded by a continuous frilled lip, having 

 a reniforni outline resulting from an indentation of its 

 ventral half. The lip is clearly divided into a long dorso- 

 lateral portion and a short sharply incurved ventral 

 portion. There is a depression at the junction of these two 



