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



given the name to the genus. It is invariably present, and 

 is anteriorly and dorsally situated, making an angle of about 

 45 degrees with the main axis. Its plane is not, however, 

 perpendicular to the sagittal plane, but slopes a little 

 towards the left. This slope is corellated with a greater 

 development of the muscles on the left side. In life the 

 surface of the disk is usually plane or convex; the concavity 

 seen in the figures is the result of contraction. The 

 youngest individuals collected (disk 7 to 9 mm. long) are 

 much depressed and attached by a broad base which is 

 parallel to the disk; in fact their shape is much like that of 

 C. macleayanum. Of the medium sized individuals (disk 

 9 to 20 mm. long), one with this depressed shape was found, 

 but the majority present what appears to be the distinctive 

 shape of the species (figs. 2, 2a). Here the region 

 posterior to the disk is largely developed, raising the shield 

 behind; and a low ridge is noticed passing from the right 

 posterior portion of the disk diagonally around the posterior 

 part of the animal and down to the left hind corner of the 

 broad base. A few of the larger individuals (disk 25 to 30 

 mm. long) still retain something of this shape; but most of 

 them have assumed an elongated or clavate form with the 

 attachment at the smaller end and the disk placed obliquely 

 on the free extremity (fig. 1). Most of the low ridge is 

 obliterated, but the anterior part persists and is often 

 sharply compressed, thus forming the " carina " mentioned 

 by Stimpson. The largest individual examined measured 

 65 millimetres in length, 38 in width (dorsoventral), and 25 

 in thickness; the disk was 40 by 26 millimetres. The 

 dimensions of an average Puget Sound specimen were: 

 length 45, width 26, thickness 21, and disk 32 by 20 

 millimetres. All the California individuals were much 

 smaller, the largest having a disk 18 millimetres long. 



The Disk. 



According to Stimpson and Drasche the disk is divided 

 into fourteen plates, but in the twenty-three individuals I 



