Counties of Durham and Northumberland. 307 



shell. The hinge-area is not so broad and more parallel than in 

 theforegoing species, and its cartilage-pits, placeddirectlyacrossthe 

 margin, and not obliquely to it, as described in the 'Monograph,^ 

 have an oblong appearance. The first pit is placed immediately 

 between the umbones, and only in one instance have I seen more 

 than three, though it is stated by Mr. King that there are some- 

 times six. This species is very imperfectly represented in the 

 ' Perm. Mon.,' for not one of the figures gives a correct idea of 

 the perfect appearance of this shell ; and the cartilage-pits are 

 represented as being oblique, which is quite contrary to what I 

 have always observed. The Bakevellia Sedgwickiana cannot be 

 considered even as a variety of this species. The surface of G. 

 antiqua and ceratophaga is, when perfect, covered with beautiful, 

 raised, distant lines ; and this is the commonest style of shell- 

 ornament that occurs in the Permian rocks. 



It is rather common in the shell-limestone of several localities. 



21. Myalina Hausmanni, Goldfuss. — It seems advisable to 

 adopt the specific name given to this shell by Goldfuss, as it is 

 the first that is accompanied with a good description and figure. 

 It has very often been mentioned, under another name, in a few 

 English works, but we have no accurate or admissible descrip- 

 tion of it, and no figure that I am acquainted with that can 

 claim earlier date than those in the ^Perm. Mon.^ The above 

 is a w^ell-established name on the continent, and has been so for 

 very many years. 



It is not possible any longer to separate this common shell 

 into two species, for there is no character by which they can be 

 distinguished specifically. The examples that occur in the lower 

 and middle beds of limestone are narrower and appear more 

 elongated than tjiose which occur in the upper deposits. The 

 latter are generally broader and more ovate in form, but in all 

 essentials they are alike. They have all occasionally the liga- 

 mental area very much produced; but this is not a specific 

 character, for individuals often occur with a narrow hinge-line. 

 The squamose appearance of these shells is not the true outer 

 surface, for the latter is most beautifully adorned with very fine 

 and regular lines of growth. It is only in finely preserved spe- 

 cimens from the upper beds that this character is shown to per- 

 fection, but it may be traced on very many specimens from the 

 shell-limestone. 



The existence of this mytiliform shell in such great plenty in 

 the shell-limestone, and its generally rough and stunted appear- 

 ance, would seem to lead to any other than the conclusion that 

 this deposit of shell-limestone was of ^pelagic' origin. The en- 

 tire fauna of this limestone is indeed so decidedly littoral in 

 character, and so clearly of shallow-water origin, that I could 



20* 



