THE ANATOMY OF PLEUKOTOMARIA BEYRICHTT. 219 



is of very little importance to P. Beyricliii, and that pos- 

 sibly it will disappear in the near future. 



The Foot. — The foot, although contracted in my speci- 

 mens, is still very large, and is evidently capable of great 

 extension. As is the case with many Prosobranchs its ante- 

 rior margin is double (6gs. 1, 2, and 7), the upper surface of 

 the foot being separated from the sole by a well-marked 

 transverse groove. We are quite at a loss to account for 

 this structure, which is evidently of great importance since 

 it is present in so many Gastropoda. 



The lateral surfaces of the foot are finely rugose, being 

 closely beset with minute papillae (figs. 1 and 2). These 

 papillae are wanting on the dorsal surface, Avliich is separated 

 from the lateral surfaces by the paired epipodial folds [ep.). 

 At the anterior extremity of the dorsal surface is situated 

 the opercular lobe (figs. 2 and 3, op. l.) ; this in its func- 

 tional condition is circular and nearly as large as the oper- 

 culum. On the right side it is produced out into a little lobe, 

 which is in turn attached to the upper surface of the foot, 

 and marks the growing point of the multispiral operculum. 

 Behind the opercular lobe a median longitudinal groove 

 leads to the posterior end of the foot ; on either side of this 

 is a modified area due to the presence of numerous transverse 

 grooves originating from the median one; some of these are 

 symmetrically arranged, but others are unpaired (fig. 2). 



This somewhat y-shaped modified area is bounded in front 

 by a couple of longitudinally-placed bands running back 

 from under the opercular lobe ; these, however, only extend 

 for about one third of the length of this area, which is else- 

 where bounded by a groove marking the commencement of 

 the epipodium. A similar modified area was found by Dall 

 in P. Adansoniana, but strangely enough this appears to 

 be quite wanting in P. Quoyana, a point upon which Dall 

 lays some stress. This is a very curious fact, for in other 

 respects, notably in the operculum and in the radula, as we 

 shall see later, P. Beyrichii is more closel}' related to P. 

 Quoyana tluiu to ^. Adansoniana, a relationship which 



