' 44 



3. discrepans. 



Mytilus discrepans, Mont. — Modlola discre- 

 pans (Lam. ?) Turt. — Mytilus discors /S. 

 Mat. and Rac. 

 At the roots of fuci ; common. 

 I have never found the shell called by some Modiola 

 nigra, and generally placed in British collections as 

 the adult of this species, on the Manx coast, though 

 the common form abounds. 



4. marmoratus. 



Mytilus discors, Mont. — Modiola discors, 

 Turt. 

 In the roots of fuci, on the northern shores. 

 I have changed the specific appellation of this spe- 

 cies, as it is not the Mytilus discors of Linn^us. 



Fam. Naiidae. 



LXIII. Unio, Retz. 



1 . margaritifera, forma Roissyi. — Unio Roissyi, 

 Michand. 



Shell oblong, thick, tumid, transversely striate ; 

 beaks depressed, acute, decorticate ; cardinal tooth 

 thick, subacute, lateral teeth obsolete. Epidermis 

 black, transversely striate. L. 2 inches ; B. 4J. 



At the meeting of the British Association in Liver- 

 pool, Mr. Gray announced that Mr. Gilbertson had 

 discovered the Unio Roissyi in Yorkshire, and Mr. 

 Wallace informs me that Mr. Gilbertson had taken 

 the Isle of Man shell also, and considered it spe- 

 cifically identical with the Yorkshire specimens. I 

 had been in the habit of considering the Manx shell 



