Zool.-Vol. I ] BANCROFT— CHELYOSOMA PRODUCTUM. 31 3 



(3) The peripheral plates, situated around the edge of 

 the disk. They vary greatly in number, and exhibit bilat- 

 eral symmetry in about half the individuals examined. In 

 the asymmetrical disks there is a tendency for the number 

 of plates on the right side to be increased. Ten cases of 

 this variation and but three of the opposite one were 

 observed. Figure 3 illustrates a case of this asymmetry. 

 Among the peripheral plates one may distinguish lateral 

 ones, the inner ends of which abut, at least partially, against 

 the central plates, and anterior and -posterior ones whose 

 inner ends reach only the siphonal plates. The lateral 

 plates are usually four in number, but occasionally, as in 

 fig. 3, additional ones are found. In some of the lateral 

 plates the corner which is next to the siphon becomes 

 extended (* fig. 3), and occasionally, especially in the 

 broader disks, it is found entirely separated and forming a 

 little accessory quadrilateral plate. The anterior plates are 

 smaller and usually more numerous than the posterior ones. 

 Occasionally the number of peripheral plates is increased 

 by the interposition of small triangular plates which reach 

 neither to the central nor siphonal ones. 



Every plate of the larger individuals has its central por- 

 tion considerably thicker than the periphery (fig. 4), and 

 this change in thickness being quite abrupt is indicated in 

 surface views by a faint line within and parallel to the 

 boundary of the plate. In extreme cases several concentric 

 series of these lines are present. On the other hand, the 

 youngest individuals examined had plates which were of 

 uniform thickness and hence exhibited none of these lines. 



Union of the Plates. 



Wagner (1885), who evidently examined the living ani- 

 mals, gives the only correct account of the relations existing 

 between adjacent plates. He says: "Die Rander der 

 dieses Integument zusammen setzenden Platten erscheinen 

 ziemlich biegsam und weich, wodurch dasselbe an diesen 

 Stellen je nach dem Willen des Thieres ausgedehnt oder 

 verengt werden kann." Sections through this region show 



