EOBSON : EXTERNAL CHARACTERS OP SINUM. 269 



posterior expansion of the propodium and the anterior portion of 

 the metapodium. The colour is a delicate brownish-pink uniformly 

 distributed on the upper and lower surface. At its widest the 

 propodium measures 25 mm. in width, the foot projecting slightly 

 beyond it. The groove separating propodium and foot-sole begins 

 some 10 mm. from the anterior end on the right-hand side, but is 

 much nearer to the anterior extremity on the other, an asymmetrical 

 arrangement which may be devoid of significance. The footsole and 

 propodium are ovoid in plan, with the posterior end emarginate 

 over the shell, in which area it covers the head and shell for a distance 

 of about 6 mm. The posterior pedal area is wider than the anterior. 

 The metapodium is not actually separated by a groove from the sole, 

 but the propodial groove is continued backwards as a ledge about 

 4-5 mm. wide on each side, that on the left reaching the posterior 

 extremity, that on the right ceasing about 10 mm. from the 

 extremity. The anterior edge of the metapodium is hollowed out, 

 and between this and the similarly emarginate posterior edge of the 

 propodium some 12 mm. of the surface of the shell are revealed. 

 The metapodium covers about 10 mm. of shell and the apex of the 

 latter lies about 2 mm. inside the metapodial border. The metapodial 

 area (inclusive of the sole) measures 34 mm. in width. The posterior 

 extremity is pointed. In its general appearance the animal is thus 

 elongate and leaf-shaped. The maximum height is 14 mm. or only 

 one-sixth of the total length. The whole of the external surface is 

 undifferentiated, and very little difference in the epithelium of the 

 plantar and dorsal surfaces of the foot is observable. 



When the shell is removed and the propodium cut back so as to 

 allow an inspection of the head it is seen that the propodium is 

 banked up against the head with an overhanging end. The head is 

 supplied with a flat, thin, semicircular cephalic shield from which 

 two slender tentacles originate. Contrary to Bergh's statement, a 

 small, rather degenerate, operculum measuring about 5*5 mm. was 

 found in this specimen. 



There ia a marked resemblance in function between the propodium 

 in Sinum and other Na'ticoids and the cephalic shield of certain 

 Bullomorpha. It would be of great interest to ascertain whether 

 the innervation of the two structures which are of different 

 origin, one being pedal, the other cephalic, is likewise different in 

 origin, as one would a priori assume, or whether, subserving the same 

 function, they are innervated from the same centres. 



The remarkable development of the foot in this genus will be best 

 illustrated by the statement that the viscera, shell, mantle, and 

 head only occupy one-seventh of the total bulk of the animal, the 

 remaining six-sevenths being represented by the enormous muscular 

 expansion of the foot. 



