312 On the Permian System of Durham and Northumberland. 



mica, De Venicuil, Perm. Mon. pi. 16. fig. 7, is only a young 

 specimen of M. elegans. 



It occurs very sparingly in the shell-limestone of Tunstall and 

 Humbleton, 



31. Edmondia elongata, Howse. PI. IV. figs. 10, 11, 12, 13. 

 " Sliell elongated oval; beaks prominent, near the anterior end; 



anterior short and rounded ; posterior elongated, the dorsal margin 

 on a line ivith the beak ; su7-face convex, covered with strong con- 

 centric lines of growth ; hinge without teeth ; mnbonal cavity di- 

 vided longitudinally by an elongated, thin, slightly curved visceral 

 plate, depending torvards the cavity of the shell." 



The above is the original description of this shell given in the 

 ' Tyneside Catalogue.' It points out one peculiarity of this spe- 

 cies and genus which appears to be generally misunderstood by 

 authors : — The shelly process situated in the umbonal cavity is 

 supposed to be a cartilage-plate, and to belong to the hinge. In 

 Mr. Woodvi^ard's excellent Manual, it is suggested, with doubt, 

 that the shells of this genus were furnished with an ossicle. An 

 examination of several cross-sections of shells belonging to this 

 genus does not appear to substantiate this conclusion. The shell 

 also of most species is very thin, and the valves are united by a 

 strong external ligament, as a Permian specimen from Germany 

 and some examples of the carboniferous E. arcuata in my posses- 

 sion clearly show. It could not, therefore, require an internal car- 

 tilage of such a size. But these are, I think, not cartilage-plates, 

 but internal processes equivalent to the subumbonal or spatula- 

 shaped blade of the genus Pholas, which projects into the cavity 

 of the shell, and forms an advanced point of attachment for a 

 visceral, or perhaps for the pedal muscle. The general configura- 

 tion of this shell also suggests that it was a burrowing mollusk. 



It is not a very common species in the shell-limestone of 

 Tunstall, Humbleton, &c., and occurs also in the equivalent 

 deposit of Poessneck. 



32. Tellina Dunelmensis, Howse. PI. IV. figs. 14, 15. 



" Shell elliptical; beaks small, not prominent, situated near the 

 anterior end, which is somewhat rounded; posterior elongated, 

 more acute than the anterior ; two cardinal teeth in each valve." 



The specimens of the shells which I have been able to examine 

 do not permit much to be added to the above characters. The 

 specimen figured in the 'Perm. Mon.' is imperfect, for the pos- 

 terior of this shell is rather acute, perhaps not quite so much so 

 as in the figures, PI. IV. figs. 14, 15. The valves are generally 

 found together, and spread out, which is the case also with 

 several other Permian species. It shows that they were provided 

 with a very strong ligament. 



In the shell-limestone at Humbleton. 

 [To be continued.] 



