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



rower at one end than at the other, straight at the back, rounded 

 at the ends, gently curved below. Excepting at the dorsal edge, 

 there is a well-defined, flattened, marginal rim. The tubercles 

 are not apparent. 



This fossil approaches very nearly in outline to L. gibbera, and 

 in some of its characters to L. marginata ; but its small size, 

 slight convexity, narrowness of shape, and large proportion of 

 marginal rim separate it from these species. 



Observations on the Genus. 



I must first observe that Prof. Quekett, having microscopically 

 examined portions of the carapace- valves of Leperditia Arctica, 

 informs me that they exhibit most distinctly characteristic crus- 

 tacean structure. 



In seeking for the family alliances of this palaeozoic genus, we 

 find some of the characters of its carapace among the existing 

 bivalved Entomostraca, both of the Phyllopod and Lophyropod 

 groups ; but others of its peculiarities are not yet traced. It is 

 not well represented by any known recent form, but partakes of 

 the characters of several. 



Shape. — The carapaces and carapace- valves figured in Plates 

 VI. & VII. present a general uniformity of shape, in the sub- 

 oval outline on three of their edges and the straightness of their 

 upper or dorsal border. They all possess the antero-dorsal 

 tubercle, and most of them show some evidence of the central 

 spot or tubercle. The ventral edge of the left valve is suddenly 

 inflected to a greater or less extent in all excepting those referred 

 to L. marginata and L. Solvensis. In the smaller form of the 

 one (PI. VII. fig. 12) there is a decided modification of the in- 

 flection alluded to; — in the latter, and in our large form of 

 L. marginata, the evidence on this point is imperfect. 



The mode in which the two valves close one on the other, by 

 a strong overlap and an inflected flange, is peculiar to the typical 

 Leperditia. 



With respect to the form of the valves among other bivalved 

 Entomostraca, fossil and recent, an approach to the outline- 

 shape of Leperditia is not uncommon ; but, except among the 

 Beyrichice, few species are definitely characterized by this exact 

 shape, with its dorsal angles and ventral curvature *. Those 

 that approach most nearly in this outline of the carapace-valve 

 are the Limnadidaf and (with the exception of being notched) 

 some of the Cypridinince. 



its convex portion has been somewhat crumpled by pressure; in other 

 respects the original form appears to be well preserved. 



* Cypridina Isabella is one of the few examples of this shape (here 

 modified by an infero -anterior notch) in other genera. 



t Especially in the young state. 



