MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



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The dorsal aponeurotic band (Figs. 1 and 2, DA) extends between the dorsal ends of these 

 areas over the dorsal side of the animal. Passing over the excavated dorsal region of the body- 

 it bends quite sharply backward as the middle line is approached. At the middle line there is a 

 pointed backward projection of the aponeurotic band. The band is about 3 millimeters in width. 



Some of the older septa show a minute cup-like backward projection at the middle of 

 their inner edge. (Fig. 1.) These depressions in the faces of the septa can be traced, constantly 

 less developed, to the newest septum. In this it is only a notch in the extreme edge of the 

 septum, into which the projection of the dorsal aponeurotic band just mentioned fits. From 

 this it would seem that in young specimens there is a projection of the mantle corresponding to 

 the depression in the septum, which has been gradually reduced until in the adult it remains only 

 as a point at the middle of the dorsal aponeurotic band. 



The ventral ends of the columellar aponeurotic areas are connected by the anterior ventral 

 aponeurotic band, which extends over the ventral surface of the body. (Fig. 1, AV.) This is also 



DAB 



AVAB 



Text-pig. 7.— Outlines of the areas of attachment of the shell muscles and aponeurotic bands to the shell spread out flat. The outline of 

 the septal area has been drawn from a wax model. 



AVAB, anterior ventral aponeurotic band. PVAB. posterior ventral aponeurotic band. DAB, dorsal aponeurotic band. SA, septal 

 area. S, siphon. SM, shell muscle. 



about 3 millimeters in width. A third aponeurotic band, the posterior ventral aponeurotic band, 

 connects the dor.sal ends of the columellar aponeurotic areas, extending also around the ventral 

 surface of the body. (Fig. 1, PV.) As the body is curved the actual position of the ventral 

 surface is posterior. The last band is only one-third as wide as the others. It is drawn some- 

 what too wide in Fig. 1. 



The posterior ventral aponeurotic band first passes directly backward from the end of the 

 columellar aponeurotic area parallel to the dorsal edges of the body. It then makes a smooth 

 curve downward and somewhat forward over the sides of the body; its median ventral portion is 

 parallel to the anterior ventral aponeurotic band. 



The posterior ventral aponeurotic band and the dorsal aponeurotic band have a peculiar 

 relation to the septal region of the body wall, since this is bounded by these two bands. In 

 most shells the aponeurotic bands and areas leave slight scars upon the inside of the shell, and 



