216 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



M. eduUs. M. latus. 



Muscles. — Anterior adductor present. No anterior adductor. 



Anterior retractor attached to the foot. Anterior retractor attached to byssus. 



Posterior retractors of the byssus Posterior retractors of the byssus 



united in one bundle. separated into two portions. 



Cavite desjiancs. — Of considerable size. No trace of such a cavity. 



Plaited orffans. — Large. Eudimentary. 



Circulatory si/stem. — Arterial system Arterial system single and median, 

 double and equilateral. 



Mantle supplied from anterior aorta ; Mantle in part supplied from posterior 



posterior aorta small. aorta, which is large. 



Margmal aiiery absent. Marginal artery present. 



Oblique vein anterior to the combined Oblique vein between middle and posterior 



posterior retractors. retractors. 



Organ of Bojanus. — Communicates with Has no communication with the peri- 



the pericardium by a large passage. cardium. 



Gills. — Ascending lamellas quite free at Ascending lamellae attached by their 



their upper edge. upper edge. 



Mantle-lobes. ■ — • Broadly joined at the Lobes joined only by a short transverse 



posterior end. bar. 



The differences in the arrangement of the muscles are, naturally, 

 indicated on the shell; that of J/, latus showing the scars of three 

 retractor muscles, a small anterior one below the ligament, a large 

 rounded middle one, and a posterior scar united to that of the posterior 

 adductor. There is, of course, no anterior adductor sear, so that, 

 in respect of adductor muscles, the mollusc is monomyarian, not 

 dimyarian. 



In M. edulis, on the other hand, there is a small but well-marked 

 anterior adductor and an anterior retractor, while the scars of the 

 middle and posterior retractors are united to one another and to 

 the posterior adductor, the combination forming a scar which has 

 somewhat the shape of the figure 6. 



Now there are many species which present the same arrangement 

 of muscular scars as in M. latus ; they include M. afer, M. variegatiis, 

 M. perna, and M. smaraqdmus, and they are the same group which 

 has been separated by the 2-1 arrangement of teeth mentioned on 

 p. 215. Presumably, therefore, the anatomical structure of these 

 species is more or less similar to that of M. latus. It is, however, 

 very desirable that other species should be examined, in order to 

 determine this, for the relative distance between the scars of the 

 middle and posterior retractors differs in different species, and some 

 of them (as Mr. Purdie has suggested) maybe intermediate in structure 

 between M. latus and M. edulis. It is, in fact, stated hj Dr. von 

 Ihering, that, in the case of 31. chorus., the separation of the retractors 

 is sometimes incomplete, " and then the impression of the [central] 

 byssal muscle is contiguous to the rest of the posterior retractor scar." ' 

 M. meridionalis seems also to be a connecting link in this and other 

 respects between the two groups. 



Dr. H. von Ihering was, I believe, the first to recognise that this 

 isolation of the middle byssal retractor and the absence of an anterior 



* Proc. Malac. Soc, vol. iv, p. 85. 



