56 



G. 0. SAES. 



No. 7. 



tides occurs, the 3 or 4 outmost of which are much larger than 

 the rest and finely spinulous on the upper edge; the denticles 

 do not however extend to the suppraanal angle but are replaced 

 on the edges limiting the anal fissure, by a very minute ciliation. 

 Besides the above mentioned marginal denticles, there is, at some 

 distance from the edge, on either side, a lateral series of about 

 6 very delicate appressed spines. The caudal claws are very 

 elongate and gently curved, having each at the base a rather 

 strong secondary denticle; they are inserted to a short promi- 

 nence occupying the anterior corner of the extremity of the tail 

 and defined posteriorly by an angular incision. 



The intestinal tube, as usual, forms in the middle part of 

 the body nearly two circumvolutions, or loops, the posterior, 

 however, being not quite perfect, but having more the appea- 

 rance of a strong S-shaped flexure. 



More generally but a single ovum or embryo is found within 

 the incubatory cavity; sometimes however there are two, and in 

 such cases, owing to the trifling width of the matrix, they are 

 generally placed one behind the other, as represented fig. 5 in 

 the following species. This, *however, only refers to the summer- 

 eggs. Of winter-eggs more than a single one is never found, and 

 this lies exactly in the middle of the incubatory cavity with its 

 greater diameter parallel to the longitudinal axis of the shell 

 (see fig. 1). The presence of such a winter-egg, easily recognized 

 by its dark opaque colour, is invariably connected with some 

 change in the structure of the dorsal part of the carapace, limit- 

 ing the incubatory cavity, without however giving rise to the 

 formation of a true ephippium. On the other hand, this part of 

 the carapace becomes very thick and more coarsely sculptured 

 than the rest of the shell, at the same time assuming by de- 

 grees a very dark bluish tint, that finally turns to black. I Q 

 specimens with summer-eggs there is no trace of this peculiar 

 colouring of the dorsal part of the carapace, the whole shell 

 being of a uniform and rather pale yellowish tint. The body 

 of the animal is generally somewhat darker changing to a clair 

 orange. 



