OF CONCHOLOGY. 85 



The rhachidian tooth is broad, rounded before, and with the 

 base arcuate behind. The cusp is simple, rounded before, and 

 more or less pointed behind. In some individuals this point is 

 more prolonged than in others, and in such cases the tip is trans- 

 parent, while the rest of the cusp is yellowish. The inner 

 twenty-seven laterals have a longitudinally arcuated rhomboidal 

 base, of which the anterior edge is produced before the cusp. 

 The latter is long, with one prominent long denticle, and one 

 short and triangular on the inner side. The tips of these den- 

 ticles are more or less transparent, while the body of the cusp 

 is yellow, and the thick base showing through is liable to mislead 

 the observer, as to the form of the cusp. The twenty-eighth 

 lateral is similar but broader, with a shorter denticle, more or 

 less bifid or arcuate at the tip, and the cusp is broadly rounded 

 on the outer side. The twenty-ninth has the prominent denticle 

 emarginate, broad and rounded, and has a third denticle on the 

 outer side. The emargination of the chief denticle is more ob- 

 vious in young specimens, and in old ones is not always notice- 

 able, except in a smaller number of the laterals. The laterals 

 gradually increase in width and decrease in length toward the 

 edge of the radula, and in the extreme outer teeth have the three 

 denticles rounded, sub-equal, and the base more than twice as 

 wide as it is long. 



The cusps of the inner laterals are, to a certain extent, bayo- 

 net-shaped, as will be seen by the figure. There are about two 

 hundred and fifty rows. 



3Iuseular System. There nre no internal bands of muscular 

 fibre as in Blelampus or Gadinia. The muscles which control 

 the buccal mass are not strongly developed. The muscles of 

 the preputium are spirally arranged in two layers. 



The adductor is divided into three parts. On the right a 

 broad passage exists where the opening of the pulmonary cavity 

 lies. The small portion of the right limb of the adductor, an- 

 terior to this, is rounded-triangular. The mantle is attached to 

 the shell, over the head, so that the scar is continuous. The 

 right extremity of the posterior part of the adductor is broadly 

 rounded. On the left the scar appears continuous, but the an- 

 terior and posterior parts, though approximate, are separated 

 by a suture, showing a tendency to bilateral symmetry. The 

 left anterior portion closely approaches the buccal mass. 



The foot is moderately thick and muscular, and divided by a 

 very faint median line in its internal muscular structure. The 

 nerves and blood vessels mostly enter the foot in this line. 



Circulatory System. The heart is enclosed in a sac, of which 



