UmTACKINUS: ITS STRUCTURE AND RELATIONS. 15 



specimen has 7 to 14 (PL V., Fig. 6); another 9 to 22 (PL VL, Fig. 5); 

 others many different arrangements, as will be seen in the table hereafter 

 given. The intersecimdibrachs differ in a similar manner. Sometimes the 

 interbrachial spaces are convex, and appear swollen, but generally they are 

 about level with the brachials and sometimes depressed. Even the proxi- 

 mal interbrachial, which Bather calls " the only stable plate," is not free 

 from variation. Sometimes it is replaced by 2, or even 3 plates (PL VI., 

 Fig. 2). In some cases it lies between the radials, and extends down 

 to the basals (PL VI., Fig. 6). There is no regularity about any of 

 these variations such as might suggest the presence of an anal side ; 

 and from what we shall see farther on, w^e know that there is none. 

 But when a large number of specimens are compared it distinctly 

 shows that the number of interbrachial plates increases with growth of 

 the individual. 



The calyx plates are very thin, usually rather flat, though sometimes 

 decidedly convex. They w^ere evidently united by a very loose suture, 

 as there is but little space on their edges for attachment of ligaments 

 or for union by close suture. The faces are sometimes marked by faint, 

 irregular, vertical grooves. The test must have been extremely pliant, as 

 well as buoyant, since in proportion to the thickness of the plates it was 

 of very large size. 



Arms and Pinnules, 



Grinnell estimated the length of the arms at one foot, or more. Hill, 

 having traced an arm fragment for a distance of 17 inches, thought the arms 

 must have been over two feet in length. Bather, from a calculation of the 

 taper of portions of arms of the British Museum specimens, came to the con- 

 clusion that in an adult the length of a free arm branch was not less than 3 

 feet 31 inches, or 100 cm. My specimens prove that Bather's estimate was 

 well founded, as we have the arm in one case actually preserved for a dis- 

 tance equal to his calculated length, with a probable addition of nearly a 

 foot more. Specimen a on Slab VIII is in this respect the most remarkable 

 that has been found (PL I., Fig. a). It was an adult individual, of nearly 

 maximum size, — the calyx being 2.50 inches, or 62 mm., wide, and com- 

 paratively quite plump. Five of its arms are preserved to great lengths, 

 — the longest one being 100 cm., or 40 inches, long, without reaching the 

 distal end. The following measurements of brachials were made at 

 different points on this arm branch : — 



