40 STUDIES IN BIOLOGY. 



seems to be occasion for separating M. latus from the others if not as 

 a genus, at any rate as a sub-genus. The following is a tabulation 

 of the points of difference : — 



Mytilus latus. 



Shell. Colour always more or less green ; in the young often 

 wholly green. 

 Hinge-teeth, one or two. 



Muscle-impressions — three retractor impressions and one 

 adductor impression. 

 Muscles. No anterior adductor. 



Anterior retractor attached to the byssus. 



Posterior retractors of the byssus separated into middle and 

 posterior retractors. 

 Cavite des flancs. No trace of such a cavity. 

 Organes godronnes. Rudimentary. 

 Alimentary canal. Pyloric caecum distinct from first part of 



intestine, extending into the mantle. 

 Circulatory system. Vascular system single and median. 

 Mantle in part supplied from posterior aorta. 

 Marginal or circumpallial artery present. 

 Posterior aorta very large. 



A single median anterior gastro-intestinal artery. 

 Posterior gastro-intestinal arteries absent, replaced by posterior 



aorta. 

 Venous system in great part lacunar. 

 Oblique vein between middle and posterior retractors. 

 No marginal sinus. 

 Organ of Bojanus. Internal communication with pericardium 



lost. 

 Gills. Ascending lamella? attached by their upper edge. 



Interlamellar junctions few, and placed singly at the middle. 

 Mantle. Mantle-lobes joined posteriorly only by a short transverse 

 bar. 



Mytilus edulis. 



Shell. Colour deep blackish-blue ; in the young the same, occa- 

 sionally whitish-yellow. 

 Hinge-teeth, three or four. 



Muscle-impressions — two retractor impressions and two ad- 

 ductor impressions. 



