99 
The Inrercostats (Pt. 11. fig. 1. to 3.) are pentagonal, one of them is in- 
serted between each of the second costals, except over the pentagonal first 
costal plate (PL. 11. fig. 2.) where on account of its angle being unable to sup- 
port a second costal plate, two intercostals adhere laterally to each other, 
On each of the five second costal plates adheres a Scapuna, (Pu. 1. fig, 
4. and 5.) having laterally two hexagonal interscapulary plates interposed, 
except over the enlarged interval, where, in the preceding series, the two inter- 
costals laterally adhere to each other over the pentagonal first costal plate, and 
where, in this series, instead of two only, three interscapulary plates intervene 
between the scapule. 
We have thus the figure of a cup completely formed, of which the arm- 
joints adhering to the scapulz, and several multangular pieces (called by me 
pectoral plates) belonging to the plated integument, form the rim. 
That all these plates must have adhered by muscular attachment and liga- 
ments, since they have no regular articulating surfaces, is evident from neces- 
sity. 'That the action of muscles extending over them, could only produce 
the pulling down of the arms and contraction of the upper portion of the ani- 
mal, may be readily supposed, and is fully proved by an examination of the 
fossil remains. The muscular force acting from the adhesion of the pelvis to 
the column as a fixed point, (PL, 11. fig. 1.and 2.) extends over the pelvis te 
the suture of the first costals, and from thence to their centres. Now it 
is evident as the pelvis consists of three pieces, and the first costal series of 
six, that three costals will be placed over the divisions of the pelvis, and three 
ever the middle of each of its plates. The muscular action exhibits hence a 
different form of contraction on the different costals arising from this variety of 
position. A simple contraction produces two folds on those over the sutures of 
the plates of the pelvis (PL. 111. fig. 9. and 17.) and a single one on those inter- 
mediate. (Pu. 111. fig. 12, and 13.) A stronger action produces four folds on 
the former (Pt. 111. fig. 11. and 18.) and three on the latter (fig. 14.) As this 
action extends to the central adhesion of the muscles on each plate, and com- 
municates from hence to its sutures and to the centres of the approximate 
plates, it produces folds somewhat similar in general disposition to those 
described in Cyathocrinites rugosus, page 90, but lessening in number as 
they approach the scapula. (Pu. 1v. fig. 14, to 26.) Irregular and partial con- 
