220 SYNOPTIC COLLECTION. 



Solarium modestum Phil. (No. 448), exhibits a much 

 closer coiling of the tube. The youngest portion of the 

 shell which is always at the apex is smooth and polished 

 (No. 448). Each succeeding whorl becomes consolidated 

 with the preceding one on the inner side, but in turning 

 round, the tube leaves so large a space in the middle 

 (called the umbilicus) that one can see nearly to the apex 

 of the shell (No. 449, S. perspectivum Lam.). 



Surcula aus trails Roissy (No. 450), looks like an inef- 

 fectual attempt to make a perfect spiral. The revolving 

 bands and lines are often broken where additions have 

 been put on at the margin. In this way the apertures of 

 the younger shells can be easily traced and the length of 

 the successive canals ascertained. This irregularity is 

 chiefly due to the anal slit in the posterior end of the 

 aperture which reminds one of the slit in Pleurotomaria, 

 Haliotis, and the like. Most of the members of the 

 family Pleurotomidae to which Surcula belongs, are with- 

 out a horny or calcareous operculum for closing the aper- 

 ture, and this is a characteristic of the deep-sea forms 

 where the struggle for existence is reduced to a minimum 

 and where therefore there is little need of protection from 

 other animals. 1 



The shell of Lunatia heros Adams (No. 451 ; see the 

 lowest shelf of the erect portion of Section 8) is a simple 

 spiral of a few turns, and is open nearly to the apex. 

 The shell is thin and covered by a yellowish brown layer 

 which is usually more or less worn off. As the shell 

 increases in size it becomes thicker. Those living on the 

 outer beaches where the waves break with force are much 

 larger and stronger than those in quiet waters. The 

 specimen (No. 451) shows the large size of the foot and 

 the mantle extending over the edge of the shell. This 

 foot can be completely withdrawn into the shell and the 

 aperture closed by a horny operculum. 



1 Ball, Bull. Mus. Comp. Zool., XVIII, 1889, p. 455. 



