exhibits a rounded oval form, with the posterior part somewhat 

 tapering and narrowly rounded off. The dorsal edge is strongly 

 arched in the middle and joins the posterior edges of the valves, 

 without any intervening angle; nor is there any angle between 

 the posterior and inferior edges, both uniting by an even, though 

 rather sharp curve. The inferior edges are nearly straight, or 

 very slightly arcuate, ascending however somewhat in their an- 

 terior part to the rounded infero-anteal corners. The head is 

 somewhat decumbent, hood-like, and, as seen laterally, rather 

 broad, with the dorsal edge evenly curved and continuous with 

 that of the carapace; it terminates with an obtuse rostrum, that 

 nearly projects in a line with the inferior edges of the valves. 

 As seen from above (fig. 6), the head exhibits a somewhat tri- 

 angular form, tapering anteriorly to an obtuse point. 



The surface of the shell exhibits a well-marked sculpture, 

 rather difficult however to observe when the animal is wholly 

 submerged in the water. On the other hand, it at once appears 

 very distinct when the shell is floating on the surface, with one 

 of its sides out of the water. The sculpture consists of a rather 

 dense striation, the striae being somewhat wavy and in some 

 places anastomosing with each other. On the greater part of 

 the carapace they have a longitudinal course, though somewhat 

 arcuate dorsally. On the anterior part of the valves, they how- 

 ever take a rather different course, ascending obliquely, so as 

 partly to run parallel to the anterior edges. In the intervening 

 space there is a partial intersection of the striae, producing an 

 irregular reticulation. A much similar sculpture is also found 

 in most of the other species and may thus be regarded as a 

 diagnostic mark of the genus. The inferior edges of the valves 

 are fringed with a dense series of rather short and delicate cilia. 



The eye is of moderate size, with the crystalline lenses, as 

 usual, rather small and but very slightly projecting. 



The ocellus is somewhat smaller than the eye and of irre- 

 gular quadrangular form; it is located at about equal distance 

 from that organ and the tip of the rostrum. 



The antennulae are of the usual cylindro-conical form an 



