94 Mr. T. R. Jones on some species of Leperditia. 



In Plate VII. fig. 14, I have figured a very interesting spe- 

 cimen, which apparently is referable to the larger form of Count 

 Keyserling's species (loc. cit. fig. 6 « to c). It is in a white 

 (dolomitic) Silurian limestone, brought by Sir John Richardson 

 from Pine Island Lake* on the English or Great River, a few 

 miles north of Cumberland House (about lat. 54°, long. 104°), 

 and is now in the British Museum. 



The fossil represents in relief the interior of a single left cara- 

 pace-valve, and appears to have been a cast which, subsequently 

 to the removal of the valve itself, has been smoothly recoated to 

 a certain extent with a thin covering, similar in colour to the 

 matrix, but less crystalline. In some aspects, the edge of the 

 cast being partially non-continuous with the matrix, the fossil 

 has an appearance of representing the valve itself, — which is not 

 the case. 



It is I inch in length and ^\ inch in breadth ; similar to some 

 specimens of L. Balthica in its obliquely suboval outline and 

 well-defined dorsal angles ; the surface is strongly convex, sloping 

 gradually posteriorly, but suddenly depressed on the anterior and 

 ventral margins to meet a well-defined flat marginal rim, which 

 ends at the extremities of the dorsal border. 



Anterior tubercle very distinct, surrounded by an irregular 

 depression ; central tubercle large, but not elevated ; radiating 

 vascular markings not apparent -, an oblique shallow furrow, 

 passing from the depression behind the anterior and above the 

 central tubercle to the most projecting portion of the posterior 

 portion of the valve, cuts off a somewhat raised area along the 

 postero-dorsal region. 



If there be any inverted plate within the ventral border (like 

 that represented in Keyserling's fig. 16 c, op. cit.), it is concealed 

 by the matrix. 



I follow M. Keyserling in placing two such apparently dis- 

 similar forms as figs. 11-14 under one specific appellation, 

 because my own materials for observation are very limited, and 

 it is possible that the Petschora-Land specimens have afforded 

 the necessary links for connecting the two by specimens of 

 different stages of growth. 



If the smaller form (figs. 11, 12) be the young of the larger 

 one, we have a carapace with a merely moulded and incurved 

 ventral edge in its young form developing a strongly bordered 

 margin with an inverted ventral plate (according to Keyserling, 

 op. cit.) in its older state ! If, on the contrary, as I am inclined to 

 suspect, the smaller form be an adult, it is necessarily distinct, 



* Journal of a Boat- Voyage through Rupert's Land, &c. 2 vols. 8vo. 

 London, 1851 (vol. i. p. 1o). 



