and pigmentation of this region, form a thickened portion that is 

 very striking in appearance when compared with similar parts 

 of most other forms. 



The radial muscles are much longer and more powerful in 

 the region opposite the hinge line than elsewhere and have for 

 their chief function the withdrawal of the mantle margins into 

 the shell in order that the margins may not be injured in closing 

 the shell, which is closed both rapidly and powerfully, especially 

 when the animal is swimming. The radial muscles of the 

 infolded ridge are continuous with these muscles and serve to 

 contract the width of the ridge or to extend the ridge out in the 

 same plane with the rest of the mantle, that is to extend it out of 

 the shell. This is no doubt done in conjunction with relaxing 

 the circular muscles of the ridge, and extending the margin by 

 blood pressure. 



The circular pallial muscles of each mantle lobe form a very 

 strong band that is attached to the corresponding shell valve 

 anteriorly and posteriorly just beneath the ridge along the hinge 

 line, at the dorsal ends of the pallial lines. They are spread out 

 between the attachment of the radial muscles and the margins 

 of the mantle lobes but are strongest some distance away from 

 the attachment of the radial muscles and they nearly or quite 

 disappear before the pallial nerve is reached. They serve as 

 constrictors that are important in withdrawing the margins into 

 the shell. In this they act in conjunction with the radial mus- 

 cles. Their attachment to the shell along the dorsal limits of 

 the pallial line suggests that they may be regarded as extended, 

 modified radial muscles of this region of the mantle. 



The circular muscles of the infolded ridges are connected with 

 this band especially near the hinge line. Here the circular 

 muscles become continuous with the other circular muscles. 

 This leaves the dorsal inch and a half or two inches of the ridges 

 both anteriorly and posteriorly without well developed circular 

 muscles. The radial muscles of the ridges of these regions are 

 also poorly developed so there is but little independent move- 

 ment of the ridges near the hinge line. It is interesting to 

 notice further that the tentacles on the ridges are not developed 

 in this region and that with the exception of the extreme dorsal 

 margin, the parts covered by the ears, the eyes are absent or 



14 



