138 CRUSTACEA. 



meeting on the back at an acute angle considerably in front of 

 the middle, and extending to the lateral margins at a point 

 deeply notched by the abrupt narrowing of the front from 

 thence to the sharp rostrum : branchial furrow forming a nearly 

 straight, delicate, impressed line from near the lower ends of 

 the nuchal furrow to the middle of each side of the posterior 

 margin (never meeting on the midline of the back) ; portion 

 in front of the nuchal furrow with a few longitudinal, 

 strong, denticulated ridges, rest of carapace rough with small 

 pointed granules : abdomen semicylindrical, large, segments 

 sculptured with rows of granules, the ends of the second 

 joint dilated, quadrate, of the others subtrigonal, penultimate 

 joint a little longer than the fifth, carrying the two outer pair 

 of tail-flaps, which are strong, truncato-elliptical, with a mesial 

 ridge, ends fimbriated, the outer one on each side divided by 

 a transverse serrated suture about one-third from the end ; 

 middle tail-flap oblong, apex truncated, narrower than the 

 base ; legs slender, compressed, smooth, gradually diminish- 

 ing in size from the first, the lower edge minutely serrated. 

 The Astacus ornatus (Phil.) is the type of this genus, which, 

 from the great compression of the carapace, size of the abdomen, 

 character and direction of the branchial furrows, &c., seems to 

 belong to the fossorial family in which I have placed it, the nearest 

 analogue being perhaps the recent Gebia which burrows under 

 the mud of Plymouth Sound : the fossils abounding in such a 

 state of perfection in the fine Speeton clay that they must have 

 lived in it and died in the exact spots we now find them, har- 

 monizes with this view of approximating them to those similar 

 little forms which live habitually buried in the mud. The sub- 

 stance of the crust, though very thin, and, in the following spe- 

 cies especially, often showing signs of considerable flexibility, 

 seems rather harder than in most of the fossorial types, and the 

 strong fringe of stifi^ hairs at the end of the tail-pieces is in the 

 fossil replaced by semi-membranous flaps, still however strongly 

 sulcated. I have not seen the extremities of the feet ; but if, as 

 I suppose, the so-called Crangon Magnevillii of Deslongchamp 

 (Mem. de la Soc, Lin. de Normandie, t. v.) belong to this genus, 

 the four hinder pair of feet would terminate in simple pointed 

 claws, and the first pair form subcheliform pincers, having the 

 hand dilated and truncated at the extremity, which is toothed 

 and has a small spiniform immoveable finger at one end, which is 

 met by the slender moveable finger inflexed from the other end ; 

 this also agrees with the general type of the fossorial Gebice. The 

 carapace may be distinguished from Glyphcea by the branchial 

 furrow in it being very delicate and extending obliquely to the 

 posterior margin without meeting its fellow of the opposite side, 



