484 Woodward and Etheridge — On Dithyrocaris. 



ward^), and led the late Mr. J. W. Salter to fall into precisely the 

 same eiTor in first describing it. 



Upon the accompanying Plate XVI. (Fig. 2), we figure Scouler's 

 original specimen by the side of that just obtained (Fig. 3), a refer- 

 ence to which will best explain the manner in which the carapace 

 has been folded and crumpled together, and the abdominal segments 

 displaced. 



The following is extracted from Dr. Scouler's original description 

 of DitJiyrocaris (Argas) tricornis : — ■ 



" The shell is elliptical but truncated anteriorly,- and much more 

 depressed than in the preceding species, and a single ridge runs in 

 the direction of the middle line. At the anterior ^ extremity of the 

 shell there are three acute triangular processes, one at each angle of 

 the shell, the one internal, separating the margin or thin portion 

 from the rest of the shell, and the other line external, dividing the 

 margin into two distinct parts. The posterior^ extremity of the 

 shell is very indistinct, and the number of joints in the tail could 

 not be ascertained," He adds : — " This specimen, as will be seen 

 by an inspection of the figure, is greatly distorted, the shell has been 

 curved, the tail or abdomen almost separated from the body. The 

 species is, however, completely distinct from the preceding. The 

 three processes at the anterior ^ extremity of the shell, the single 

 ridge running along its middle, are sufficient to distinguish it." 

 (op. cit. p. 138.) 



The following measurements are taken from Dr. Scouler's speci- 

 men (not copied from his description, which gives them rather in- 

 accurately). The carapace measures 3;^ inches=80 mm. ; breadth 

 of carapace, 1-| inches^38 mm. ; length of caudal segment, 1 inch=: 

 25 mm. ; breadth, 15 mm. ; length of longest tail-spine, 40 mm. 

 From an examination of Dr. Scouler's original specimen by one of 

 us (H. Woodward), the impressions of the maxilla (see Plate XVI. 

 Fig. 2 g) near the anterior extremity of the carapace have clearly 

 been made out. 



The projecting portion at the anterior extremity (see Fig. 2 a) is 

 no doubt a part of the folded margin of the shield. Three segments 

 are observable in the abdominal or caudal series (c). 



As before stated, Dr. Scouler's specimen has been completely 

 decorticated, and being in a harder and more compact matrix, has 

 undergone a greater amount of mineral change than the specimen 

 obtained by Mr. Bennie. 



This example (Fig. 3) presents us with an almost perfect dorsal 

 aspect of the carapace fully spread out upon the shaly matrix, and 

 still retaining the delicate and peculiar ornamentation of the surface 

 (see Plate XVI. Fig. 3). 



The carapace is less rounded than in the preceding species (D. 

 testudinea) ; the outline of the lateral margins is elliptical, the 



^ See Murchison's " Siluria," 4th edition, 1867. p. 236, fig. 66, and footnote. See 

 also Geol. Mag., 1865, Vol. II. p. 401, PI. XI. fig. 1. 

 * For '^anteriov" read post f.rior. 

 ^ For " posterior" read anterior. 



