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



Right valve nearly oblong, broadest and roundest behind ; 

 tapering and angular in front; most convex in the middle; 

 depressed towards the margins, which are for the most part 

 slightly raised; ventral border overlapping the opposite valve 

 to the extent of ^th of the breadth of the carapace ; anterior 

 tubercle distinct ; central spot not defined. Surface smooth, not 

 polished; black. 



Another, but smaller, right valve (younger individual) is simi- 

 lar in general form, but relatively shorter and broader. 



These two specimens occur on a small fragment of dark, 

 shelly, Devonian limestone from Nehou in Normandy, which 

 has been kindly lent me by Mr. Morris. 



It appears to me probable that they belong to M. Rouault's 

 species (which is from the Devonian rocks of Brittany), although 

 the latter form appears to differ in the degree of convexity, and 

 to be shorter and broader in proportion, but unfortunately 

 neither M. Rouault's specific description nor figures supply me 

 with all the necessary terms of comparison. 



The Normandy specimen, PL VI. fig. 6, differs from the right 

 valve of L. Balthica in its narrower and more angular outline ; 

 in the greater depression of the surface towards the margins of 

 the valve, or more rapid slope from the centre towards the peri- 

 phery ; and in the greater overlap of the ventral margin. 



The vertical transverse section of the closed valves oi L. Bri- 

 tannica, fig. 7, is copied from M. Rouault^s memoir for the sake 

 of comparison. The ventral overlap of the larger valve is con- 

 siderable, as in fig. 6 a ; but the convexity of that valve differs in 

 the two individuals. The inverted plate of the venti-al border of 

 the smaller valve, as shown in fig. 7, may be remarked as being 

 of very small extent, compared with that of L. Balthica, and of 

 some of the species hereafter described. In M. Rouault's fig. 1, 

 the small valve is seen to have a dorsal protuberance, like that 

 in L. gibbera (PI. VII. fig. 9 a) ; hence, if I am right in assign- 

 ing the Normandy specimens to the Bretagne species, their left 

 valve would have the dorsal hump, — another characteristic fea- 

 ture distinguishing them from L. Balthica. 



5. Leperditia gibbera, nov. sp. PI. VII. figs. 8-10. 



Dimensions of the separate valves of four individuals : — 



Length. Breadth, 



(inch) (inch) 



2% 2% I^eft valve. 



8 4 



Right valve. 



2¥ 2'i -^"O* 



\ ^ Right and left valves. 



Valves strongly convex ; almost oblong, broadest at the pos- 



