250 NOTES ON THE PERMIAN SYSTEM OF 



rise, and seem to liave been for the more secure attachment of 

 the mantle and for giving it a greater surface. 



In the lower valve an elevated callosity near the beak of the 

 shell forms a fulcrum or base for the attachment of the adductor 

 muscles ; its surface is strongly rugose. On each side of it are 

 placed the somewhat oblong, finely-striated impressions of the 

 cardinal muscles. The small pittings on the ovarian regions, 

 the spinose callosities, and the grooved surface of the interior, 

 agree with the same appearances in the upper valve. 



It is found in England in the compact, and shell-limestone 

 only, in numerous localities. 



2. P. LATIROSTRATUS, HoiCSe. 



When I was engaged in drawing up my Catalogue of Per- 

 mian Fossils, I found that this shell had not been described or 

 mentioned by any one ; and as I had collected several fine speci- 

 mens at Dalton-le-dale, and had not seen or heard of any other 

 specimens than these, I concluded that it was entirely new, and 

 described it as such in the " Tyneside Transactions" as follows:— 



Shell gri/pJueoid, concavo-convex ; loiver valve convex^ bilobed, or 

 ivith a slight furroiv or sinus in the centre; vpijer valve slightly con- 

 cave, or nearly flat ; heah of lower or convex valve large, and very 

 much flattened; hinge-line of upper valve rather angulated, furnished 

 with a large triangular button; surface of convex valve covered ivith a 

 few distant spines. 



The above description was drawn up from a cast, PL XL, fig. 

 2, which Mr. Davidson has recently figured, Mon. Brit. Brach., 

 pi. 4, fig. 5, and which Mr. King had the loan of, with all my 

 other Permian Producti, for a considerable time. The discovery 

 of a large series of specimens at Tunstall in a fine state of pre- 

 servation enables me, through the liberality and kindness of 

 Mr. Kirkby, to add a few more particulars to the above descrip- 

 tion. 



The general form is subquadrate, somewhat compressed when 

 seen laterally, but in full-grown specimens it has a considerably 

 produced frontal margin. The lower valve only is covered 

 sparingly with spines of great length ; some that I have seen 



