130 MYTILIDJE. 



coast, one of them measuring nearly two inches and 

 a half in length. 



Chemnitz and Fabricius considered it to be a variety 

 of M . discors j and Montagu noticed it as a large form 

 of his Mytilus discrepans. Leach, in his ' Zoological 

 Miscellany, 3 excellently described it by the last-men- 

 tioned name; and he referred the Devonshire speci- 

 mens of M. discors to a small variety of the same species. 

 This was many years before Gray's publication; and 

 perhaps, in strict justice, the name of discrepans ought 

 to be restored and applied to the present species. But 

 I will mercifully abstain from increasing the perplexity 

 which has so long involved the nomenclature of the 

 Modiolaria. 



Genus III. CRENE'LLA*, Brown. PL III. f. 4. 



Body roundish-oval : mantle open in front, and folded be- 

 hind into a sessile excurrent tube : foot worm-shaped, the 

 point being disk-like and issuing out of a sheath. 



Shell oval or rhoniboidal, cancellated by longitudinal ribs 

 and transverse plates: heaks straight: ligament small: Mngi 

 of each valve furnished with an upright tooth, which is crenu- 

 lated as well as the hinge-plate. 



The name of this genus was probably derived from 

 the circumstance of the hinge-plate being slightly 

 notched. The shell is entirely composed of nacre, and 

 has a silvery hue when deprived of the epidermis : in 

 this respect it resembles that of Nucula. The animal is 

 unlike that oiModiolaria. It has the mantle completely 

 open in front, instead of being folded into a special tube 

 for the entry of food and water; the excretory tube is 

 exceedingly short, and sessile, instead of being produced 



* A little notch. 



