750 mantle; shell; nuchal membrane. 



of the muscles or briefly the muscle-lines, in contradistinction to the growth-lines of the 

 shell, or simply, shell-lines. Careful inspection will further reveal the fact that what 

 I have called the muscle-lines are not confined to the actual muscle-scar, they do not 

 merely represent the symmetrical impressions of the muscle-fibres, but the concentric 

 lines are continued behind the scar as far as the septal suture, thus clearly indicating 

 a progressive forward movement of the muscles in correspondence with the growth of the 

 entire animal. 



It is important for a just appreciation of these details to note that the muscle- 

 lines on the shell are concentric with the anterior border of the muscular impression, 

 and have no relation to its posterior border, although the latter has a definite contour 

 rendered visible in spite of the continuity of the muscle-lines across and behind it, by the 

 fact that there is a glistening pearly sheen in the region of the muscular impression, 

 and merely a dull lustre behind it, though it must be added that this distinction is 

 more noticeable in some specimens than in others. (PI. LXXVI. fig. 9.) 



The nacreous deposit which overlies the shell-lines behind the anterior border of the 

 muscular and annular impression, and is continued upon the face of the septum, has been 

 interpreted as hypostracum, the rest of the pearly substance of the shell being the ostracum, 

 while the external porcellanous pigmented layer is the periostracum 1 . 



Keferstein (1865) and Appellof (1893) supposed that the mechanism of the forward 

 movement of the muscles in the shell consisted of a resolution of muscular substance 

 at the hinder border, coincident with a formation of fresh muscular substance in front. 

 But this pretended resorption of muscle-fibres could not be confined to the ends of the 

 muscles where they abut upon the shell, but must affect the entire body of the muscles. 

 There is no evidence whatever that anything of the kind takes place since the muscles 

 increase in size pari passu with the growth of the animal, and the presence of the 

 concentric muscle-lines on the shell, visible as they are from the septal suture to the 

 anterior border of the muscle-scar on each side, is clearly indicative of a very gradual 

 forward gliding of the animal. As the animal grows it must of necessity move forwards 

 within the rigid walls of the shell, since the increase in size takes place in every direction, 

 quite as much in girth as in length. At the same time the soft visceral sac can 

 accommodate itself to a certain extent to straitened circumstances, sufficiently to avoid 

 any sudden catastrophic movement, and meanwhile gas is secreted by or through the 

 thin septal area of the mantle, and when the limit of growth at any particular period is 

 reached, a new septum is laid down in the manner which I have described above. As 

 the animal moves forward over a tract equivalent to the interval between two successive 

 septa, a calcareous deposit is formed between the old and the new septal sutures. 



With regard to the relation subsisting between the septal aponeurosis and the annulus, 

 or to keep uniformity of nomenclature, the annular aponeurosis 2 which secretes the horny 

 girdle on its surface, it is to be noted that, on the dorsal side of the mantle these 

 structures are more or less confounded together, while ventrally there is in adult 



1 Thiele, J., " Beitrage zur Keuntnis der Mollusken. II. Uber die Molluskenschale." Zeitschr. wiss. 

 Zool., Bd. 55, 1893, see p. 234 with text-figure. In the figure the author has omitted to represent the 

 posterior contour of the actual muscular impression. 



2 Valenciennes likened the annulus to an aponeurosis. 



