16 Gr. O. Sars. 



Seen laterally (fig. 4), the shell exhibits a somewhat irregu- 

 lar, obliquely oval form, being gradually expanded posteriorly, 

 and terminates in a well-defined, though rather short and blunt 

 prominence, which extends in the direction of the longitudinal 

 axis of the body. The dorsal margin appears, throughout 

 the greater part of its length, almost straight; but quite 

 posteriorly it makes an abrupt bend, so as to form here a 

 greatly projecting gibbous expansion, extending just above 

 the median prominence, and defined from it by a well-marked 

 sinus. The edge of the expansion is coarsely denticulated, 

 the denticles being also continued around the median pro- 

 minence The inferior edges of the valves appear somewhat 

 less bulging in front, and join the oblique posterior edges 

 by a greater curve. 



The head (see also fig. 5) is comparatively larger than 

 in the preceding species, and less procumbent, being, as 

 usual, defined from the shell above by a small indentation. 

 Its dorsal margin is slightly curved, and joins the ventral 

 edges immediately, without any intervening frontal projection, 

 the front itself being narrowly rounded. The ventral edge 

 is slightly convex in the middle, and scarcely ascending, 

 and the rostral projection is rather indistinct, and not, as 

 usual, deflexed. The fornix exhibits the usual appearance. 



The eye (see fig. 5) is comparatively somewhat larger 

 than in the preceding species, and located just within the 

 front. 



The ocellus (ibid.) exhibits an appearance similar to 

 that found in S- Elisabethce, and iu the European species 

 S. vetulus, being drawn out in front to a narrow stripe. 



The tail (fig. 6) is not nearly so broad as in S. acu- 

 tirostratus, the posterior edge above the anal sinus not 

 being, as in that species, gibbously expauded, but only very 



