5 STUDIES IN BIOLOGY. 



Sections taken vertically through the two valves (Fig. 7) are 

 somewhat rounded in the anterior third of the shell (Fig. 8, a), 

 ovate about the middle of the shell (Fig. 8, b), and lanceolate in the 

 hindmost third (Fig. 8, c), the wider part of these sections being 

 ventral. 



Impressions of the Muscles. (Figs. 4 and 5.) 



Taking first the impressions of the adductor muscles, the posterior 

 adductor impression is similar in M. latus, edulis, and magellanicus. It 

 is a large, distinct oblong or elliptical impression on the upper and 

 hinder part of the shell, distant somewhat less than one-third of the 

 length of the shell from the posterior end. (Figs. 5, 6, and 19, 

 p ad.) 



Anterior adductor. In M. edulis and magellanicus this forms a 

 small oval impression on the ventral side at the fore-end of the shell, 

 placed just where the pallial line ends {a ad, Fig. 5) . In M. latus 

 there is no trace of the impression of an anterior adductor (Fig. 4). 



Taking next the impressions of the retractors, I shall not now 

 discuss whether they be retractors of the foot or of the byssus, but 

 shall call them indifferently retractors. 



Anterior retractor. This is alike in the three species as regards 

 its impression, which is oval and placed close under the hinge at the 

 anterior end of the shell [art, Figs. 4 and 5). 



Posterior retractors. With respect to the impressions of these 

 muscles, a considerable difference obtains between M. latus and edulis 

 or M. magellanicus. In the two last there is an elongate impression 

 just in front of and continuous with the posterior adductor impres- 

 sion. This retractor impression is not quite as continuous in M. 

 magellanicus as in M. edidis, but the arrangement is the same in both 

 (p rt, Fig. 5) . 



In M. latus there is a short muscle impression just in front of 

 and continuous with the posterior adductor impression (prt, Fig. 4) ; 

 but there is in addition, midway between this and the anterior re- 

 tractor impression, a very distinct and somewhat large oval impres- 

 sion (m rt, Fig. 4) . 



These two impressions of M. latus, if brought together, would 

 form an elongate impression exactly proportionate to that of M. edulis 

 and magellanicus (Figs. 4 and 5). The first thought is that the con- 

 tinuous muscle impression of these two species has been divided in 

 M. latus, part of the muscle being moved forward; and when the 

 muscles themselves are examined this is found to be the case. I 

 have called these middle muscles the " middle retractors :" hence in 



