GENUS NEBELA— NEBELA HIPPOOREPIS. 157 



ward and inward into the cavity. The whole extent of this peculiar 

 appendage to the shell occupies about two-thirds its length. Its projecting 

 digit-like ends divide off from the general cavity of the shell a pair of 

 elongated conical recesses extending upward and outward along the lateral 

 border. At the fundus, the keel is about half the thickness of its depth, 

 but becomes thinner to its termination. It is pale straw-color and indis- 

 tinctly granular. 



The structure of the shell, independent of the keel, is the same as in 

 the other species of Nebela; but, in the few specimens observed, it was in 

 all composed of circular disks of nearly uniform size 



The sarcode has the same color, relations, kind of attachment, and 

 structure as in N. collaris. A clearer space in the centre of the endosarc 

 indicates the presence of the usual nucleus. 



When Nebela hippocrepis was disturbed, ordinarily it would withdraw 

 the pseudopods, but the sarcode mass would remain connected with the 

 mouth of the shell. When obliged to leave the latter in receding, it would 

 sometimes retain a connection with the ends of the horseshoe-like appendage 

 of the shell, as seen in fig. 14. 



The smallest specimen observed was ~t\\ of an inch long, ^th of an 

 inch broad, 3 -J_th of an inch thick, and had the mouth ^th by -~^h of an 

 inch. The carina was -^th of an inch deep at the fundus, and had the 

 digit-like ends ith of an inch long. 



The largest specimen was ~\h of an inch long, ith of an inch broad, 

 ith of an inch thick, with the mouth i.th by ~\\\ of an inch, and the carina 

 was ^th of an inch deep and -^th of an inch thick. 



In two instances I found specimens of empty shells of a Nebela, 

 evidently related with that described, though they did not possess the 

 horseshoe appendage. The shells had the same shape and structure as N. 

 hippocrepis, but were smaller. In one, represented in fig. 13, pi. XXIV, 

 two long conical horn-like recesses, or hollow processes, extended from the 

 lateral borders of the fundus downward into the cavity of the shell. In 

 the other specimen, the horn-like recesses were interrupted in their course, so 

 that the lower third was separated from the upper portion. If these horn- 

 like recesses wei'e solid and continuous by a horseshoe appendage in the 

 interval around the fundus of the shell, we would have the condition 

 ordinarily presented by Nebela hippocrepis. 



