TRACHYSOMA. 41 



the carapace being altogether more tumid, the granulations on the surface less regular 

 and universal in their distribution, the cervical furrow deeper, and extending nearer 

 to the margin of the carapace, and the hepatic furrow of a somewhat different form. 

 As only a mutilated carapace of this character exists, I do not presume to treat it 

 as a distinct species, and most probably it is, after all, only an accidentally abnormal 

 condition of H. Belli. 



Genus — Trachysoma, Bell. 



Trachysoma scabrum, mild. Plate X, fig. 11. 



I have found it necessary to assign a distinct generic position to a form which differs 

 from all others that 1 have seen, although, unfortunately, the only example of it yet 

 discovered is a single specimen in Mr. Wetherell's collection, which consists exclusively 

 of a broken and imperfect carapace, and a few undistinguishable fragments of the limbs. 

 From these scanty data I can only give slight and provisional characters, which, however, 

 with the figure, may be sufficient to enable future observers to identify the species, and 

 more fully to describe it from more perfect specimens. 



The carapace is long, apparently flattened at the sides (though this appearance may 

 possibly arise from pressure, of which there are other unmistakeable indications), and the 

 whole surface highly scabrous ; the cervical furrow is deep and strongly marked, and runs 

 in almost a straight line directly across the carapace ; the " cephalic arch " or portion 

 anterior to the cervical furrow, comprising the gastric and hepatic regions, occupies about 

 one third of the length of the carapace, and is longitudinally marked with strong ridges 

 and intermediate furrows, the latter being smooth, whilst the ridges resemble the general 

 surface in being very rough, with numerous small, sharp tubercles ; the " scapular arch," 

 formed of the cardiac and branchial regions, has a diagonal furrow, which separates these 

 two regions ; the posterior margin is waved, and, like the lateral, has a distinct, raised 

 border. A considerable piece of the antennal scale remains, which shows that it 

 partook of the scabrous and almost spinous character of the other parts; and there 

 are some fragments of several of the thoracic members, one of which appears to be 

 a portion of the arm, which is less rough than the carapace, and one piece, which is 

 probably a portion of one of the ambulatory legs, is quite smooth and polished. 



Length of carapace, 1*3 inch. 



I do not presume, upon such an imperfect basis, to offer any present suggestions 

 as to the affinities of this species. It differs in so many respects from every known form, 

 either extinct or existing, that we must wait for further data before any satisfactory 

 opinion can be adopted. 



The specimen was found in the tunnel near Chalk Farm. 



6 



