Woodward and Etkeridge — On Dithyrocans. 485 



latero-anterior angles are rounded, the front border only slightly 

 depressed near the centre ; ^ the posterior angles curve slightly 

 inwards, and are then produced into two pointed spines (10 mm. in 

 length), one at each angle ; the hinder border is truncated, and was 

 doubtless furnished with a median spine, as seen in Scouler's original 

 specimen. Fig. 2p, but it is not preserved in this example. 



The general surface of the carapace is marked by minute angular 

 plicEe. The dorsal ridge extends from the posterior border for three- 

 fourths of the length of the carapace, and is marked by numerous 

 small but elevated transverse wrinkles. 



Two ridges similar in ornamentation to the dorsal ridge arise 

 about 4 mm. distant on either side of the median line, for one-third 

 of their course ; passing from near the frontal border, where they are 

 more strongly marked, they continue in a sinuous line towards the 

 posterior margin of the carapace, and are only faintly seen for the 

 remaining two -thirds of their length. 



Two medio-lateral ridges, distant about 15 mm. from the dorsal 

 ridge, traverse the carapace on either side, forming two very promi- 

 nent and strongly marked lateral keels, which stand out in bold 

 relief 4 mm. above the general surface of the shield, and are orna- 

 mented with similar transverse wrinkles. Two short and curved 

 ridges (Fig. 3 s), also ornamented by transverse wrinkles, 10 mm. in 

 length, mark the summits of two well-defined rounded prominences, 

 situated on either side within the frontal border of the carapace, 

 10 mm. from the dorsal line. Each ridge resembles in curve a long 

 f, being rounded and deeply circumscribed at its posterior extremity, 

 which is marked on the summit by a minute transverse reniform 

 depression 1 mm. long. These short ridges (Fig. 3 s) doubtless 

 serve internally as points d'aj)pui for the attachment of the maxillary 

 muscles. 



The external border is in great part removed, displaying the finely 

 striated ventral margin ; but where it is joreserved, it exhibits the 

 same oblique striation described by M'Coy as ornamenting the 

 border of his D. Scouleri.^ The margin is well defined, and about 

 two mm. in breath. 



There is evidence upon both the specimen and counterpart that 

 the integument composing the lateral margins of the carapace was 

 double, as in the shield of recent Apus, Limidus, and other Crustacea ; 

 the infolded border being 10 mm. wide near the middle of the 

 margin, and very delicately striated upon its sui-face. (A minute 

 specimen perhaps belonging to this species (D. tricornis), 15 mm. in 

 length, and 10 mm. in breadth, exhibits the same infolding of the 

 lateral margins as the large example displays.) 



There are remains of three caudal or abdominal segments, and 

 parts of the tail-spines, the former ornamented with transversely 

 divaricating raised lines, the latter with longitudinal striae. Length, 



1 Not deeply emarginated, as in B. Scouleri, M'Coy. 



2 M'Coy, Carb. Foss. Ireland, p. 163, pi. xxiii. fig. 2. There seems no reason to 

 doubt that this species is synonymous with J), testudineus, with which it should now 

 be merged. 



