BOTHBIOLEPIS LEPTOCHEIRUS. 123 



Specific Characters. — Plates of carapace thin, sculpture as in II. major, hut 

 relatively rather finer; median dorsal plates not keeled; pectoral appendages long 

 and slender; no prominent denticulations observable on their outer margins. 



Description. — The remains of this species occur, like those of the larger species 

 of Bothriol&pis generally, in a dislocated condition, the only elements found in 

 apposition being those of the upper arm, which may also still remain in conjunction 

 with the anterior ventro-lateral plate. Such a specimen is seen, natural size, in 

 PI. XX[X, Fig. 3, which represents the outer surface of an almost complete 

 example of the right (interior ventro-lateral plate, with the proximal part of the 

 pectoral limb attached. The bony matter of the plate is relatively thin, but the 

 external sculpture is seen to present the same tuberculo-reticulate character which 

 we have already seen in B. major, though relatively more delicate than is usually 

 the case in the last named species. Attached to the articular fossa of this plate is 

 the proximal segment of the arm, but split through the middle so as to show the 

 internal surfaces of the plates forming its dorsal aspect. We are struck with the 

 proportionally long and slender appearance of this upper arm, compared with the 

 same part in other species, and this character is also shared by the distal portion 

 represented in Fig. ■). Fig. 4 shows the isolated articular element of the arm, 

 with an imperfect posterior median-dorsal plate on the same portion of stone. The 

 former, seen from the external surface, is proportionately more slender than in 

 II. major (see PI. XXVI, Figs. 2 and 3), as its greatest breadth is contained three 

 and a third times in its length, whereas in B. major the proportion is as one to 

 three. In other examples of the upper arm of II. leptocheirus the proportion of the 

 breadth to the length of this articular element is as one to three and a half. No 

 specimen of the arm shows any prominent denticulation of the external marginal 

 element, the species agreeing in this respect with II. major. 



As to the peculiar proportional slenderness of the arm, on account of which I 

 have given the name of leptoclieirus to the present species, there can be no doubt, 

 but at the same time I rather think that this slenderness is more due to propor- 

 tional narrowness than to excessive length. 



In Fig. 2 we have a portion of the outer aspect of the upper end of an arm. 

 showing the junction of the two articular elements, dorsal and ventral, over the 

 proximal end of the external marginal. 



Fig. 1 shows the premedian plate of the top of the head with the sensory groove 

 crossing it just behind the anterior margin. In form and in general aspect it 

 scarcely differs from that in II. major. 



Returning to Fig. I-, we have noticed that it displays, besides the already 

 described articular plate, a considerable portion of a posterior median dorsal. Tin- 

 is seen with the posterior extremity and margin directed upwards; such of the 

 bony matter as is present is viewed from the internal aspect, and where it lias 



