242 Mr. W. King on some Shells and other Invertebrate Forms 



Axinus flexuosus = genus Cryptodon, Turton = PtycMna, 

 Philippi = Lucina, auct. 



This shell appears to be much rarer on the east than on the 

 west coast of Britain. Professor Macgillivray has found it off 

 Aberdeenshire, and Mr. Maclaren has procured it on the Ber- 

 wickshire coast. I have only seen a single specimen belonging 

 to Northumberland, and that came up on the lines after they had 

 been down in thirty fathoms water, twenty-five miles east of the 

 Fern Islands. 



If this shell must be separated from the genus Lucina, it will 

 have to be named Axinus instead of Cryptodon, as the former 

 name was previously applied to an allied or congeneric fossil 

 {Axinus angulatus) belonging to the London clay, Mr. J. Sowerby 

 having published the genus so designated in December 1823 (the 

 date of No. 55 ' Mineral Conchology/ in which it first appeared), 

 while that of Cryptodon was not published till the early part of 

 the following year (vide the dates of the dedication and title- 

 page of Turton's < British Shells')*. 



Mysia undata (Leach's genus) = Venus undata, Pennant. 



I dredged a specimen of this shell in fifty fathoms, but it is 

 also to be found in much shallower water, as it is occasionally 

 taken on the lines that have been down in twenty and thirty fa- 

 thoms. Mysia undata and Diplodonta rotundata have often been 

 placed in the same genus : the sinus in the pallial line of the 

 former, however, generically separates it from the latter, which is 

 one of the Lucinidce. 



Mya truncata, Linnaeus. 



Variety M. pelagica, nob. This variety is from deep water off 

 the coast of Northumberland : it resembles the ordinary form of 

 Mya truncata, but is more truncated posteriorly, approximating 

 in this respect to Mya uddevallensis ; but instead of the trunca- 

 tion being oblique as in the latter, it is perpendicular as in the 

 former. Further, Mya pelagica agrees with the normal form of 

 Mya truncata in the curve of the pallial sinus, but differs from it 

 in the position of the posterior adductor muscular impressions, 



* Mr. J. Sowerby included in the genus Axinus a very different shell 

 belonging to the magnesian limestone — the so-called Axinus obscurus, for 

 which and some mountain limestone species I have formed the genus 

 Schizodus (vide'Sir Roderick Murchison's Geology of Russia, vol. ii. p. 308). 

 Professor E. Forbes, in stating that this shell " was the type of the Sowerbian 

 genus Axinus" (vide vol. i. Memoirs of the Geological Survey, p. 412), over- 

 looks the express declaration of Sowerby himself, that the London clay spe- 

 cies {Axinus angulatus) was to be considered the type of this genus : this 

 shell is also ti*e one first described. 



