MOIITHUMBEIILAND xVND DURHAM. 268 



ance. The first pit is placed immediately between the iim- 

 bones, and only in one instance have I seen more than three, 

 though it is stated by Mr. King that there are sometimes six. 

 This species is very imperfectly represented in the " Perm. Mon.," 

 for not one of the figures gives a correct idea of the perfect ap- 

 pearance of this shell ; and the cartilage pits are represented as 

 being oblique, which is quite contrary to what I have always 

 observed. The Bakevellla Seclgwickiana cannot be considered 

 even as a variety of this species. The surface of G. antiqua and 

 ceratopliaga 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. 



17. MYALINA, Kouinck. 



] . M. 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 been mentioned, under 

 another name, in a few English works, but we have no 

 accurate or admissible description 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 well-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 those 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- 



