Mr. T. R. Jones on some species of Leperditia. 83 



— the relative thickness and the horny aspect of the valves, which 

 are marked moreover by a small tubercle, and by a central spot, 

 formed by a local variation in the tissue of the test. 



With these characters, it is readily recognized as a Crustacean ; 

 but with regard to the place of the Gothland fossil amongst its 

 congeners, although the several characters above enumerated are 

 individually represented by other Bivalved Entomostraca, yet 

 there is no genus, either existing, or yet described from the 

 tertiary or secondary rocks, to which this fossil can be referred. 

 The genus Beyrichia, of the Silurian rocks, presents some special 

 characters in common with the fossil in question ; but the two 

 forms are unmistakeably distinct *. 



In the description of some fossils from the Devonian schist 

 and limestone of Ille et Vilaine (Brittany), M. Rouaultf in 1851 

 recognized two new Entomostracan bivalves — a Beyrichia {B, 

 Hardouiniana%j Rouault), and a form on which he founded the 

 genus Leperditia, The generic characters of this Bretagne spe- 

 cies (L. Britannica, Rouault) are clearly described, and are closely 

 applicable to the Gothland fossil above referred to. 



Through the kindness of Mr. J. Morris, I have had the op- 

 portunity of studying some very fine specimens of the Gothland 

 form, as well as two valves from Nehou, which are referable 



find, and indeed do not know of, any genus bearing the same characters 

 united as in your Crustacean, namely a straight hinge-Une and an over- 

 lapping ventral margin. 



" The following genera show more or less one or other of these cha- 

 racters : — 



Solemya, 



Lithodomus, 



Modiola, 



Mycetopus, x ^ • i ^ i • v 



Iridina, f ^ «*^^g^* hmge-hne. 



Area, 



Meleagrina, 



Perna, 



Avicula, inequivalve, with straight hinge-line. 



Potamomya, T 



cImu, \ ^^ overlapping ventral margin. 

 Thracia, J 



" The dorsal margin of Solemya, with the ventral margin of Potamomya, 

 would, I think, produce the form required, but such a form is unknown to 

 me." 



* Vide antea, vol. xvi. pp. 91 & 174. 



t Bullet. Soc. Geol. France, 2 ser. vol. viii. p. 377- 



X This species was inadvertently overlooked when I was preparing the 

 Table o{ the Beyrichia:, antea, vol. xvi. p. 175. 



I take this opportunity of correcting an erratum in the Table here referred 

 to : — B. Wilckensiana, and its \2iV\tiy plicata, should be placed under the 

 heading Corrugatse, instead of under Simplices. 



6* 



