52 BRITISH FOSSILS. 



are much smaller than in H. bilobus, being not above one-fourth the 

 length of the carapace, and very convex. 



The body is at first wider than the head and then tapers back- 

 wards. The first ring is very narrow (fig. 42), the second twice as 

 broad and with the usual dilated extremities ; the third, fourth, and 

 fifth strap-shaped, arched in the middle and direct on the sides so 

 that the segment appears much bent. The ends are truncate, in the 

 anterior rings widest behind, and in the posterior ones tapering 

 backwards. 



Fig. 28 shows one of the hindermost rings, such as are seen in the 

 more complete specimen, fig. 23. The hinder rings become gradually 

 less transverse, the tenth only two and a half times wider than long, 

 and the penultimate about once and three-quarters its own length. 



The caudal joint (telson) figs. 23, 38, 45, differs, in its expanded 

 form, materially from that of H. bilobus. It is about three-fourths 

 as long as wide, narrow at the base with two short ridges running 

 down from either angle ; then expanded with somewhat convex 

 sides towards the wide subtruncate apex ; the outer angles are 

 rounded off, the terminal notch shallow, and a short median keel 

 continued from it one-third up the segment. 



The sculpture of the head is not known. On the body rings a 

 transverse lineation running into open plicae on the sides (fig. 43 a) 

 occupies the front margin for not quite half the segment ; a few 

 plicse are intermixed with the lines. 



[On the caudal joint, fig. 46, a lineation, parallel to the outer 

 border (a) is distinct, but it is uncertain if this specimen be of the 

 same species ; it has a strong median groove down the under side, 

 and is less expanded in form than P. Banksiii] 



The swimming foot, fig. 42, has a characteristic shape, the upper 

 joints (fourth and fifth) are rather narrow, and the penultimate (p), 

 instead of being simply conical as in H. bilobus, is ovate (more so 

 than in our figure), with the outer border especially convex. It is 

 notched above to receive the fifth joint, and below divided into very 

 unequal lobes. The terminal palette d is true oval, rather blunt 

 at its origin and more pointed at the extremity. It is nearly equal 

 to the penultimate joint in length, but considerably narrower. 



Fig. 35 is a penultimate joint, probably of the same species. 



Antennce. Figs. 30 and 40 are presumed to belong to this species, 

 as they occur with them. They are remarkably slender and straight ; 

 the base is large and broad, suddenly attenuated into the shaft, 

 which is only a tenth of an inch wide, and three-quarters of an inch 

 long, beset with close small teeth, and furnished with three larger 



