286 THE CANADIAN NATURALIST. [Sept. 



exceedingly thin membranous shell that composed the walls of 

 the tubes. The number of pores varies with the size of the 

 individual. In large specimens these are from 800 to 1000. 



It has been stated by some authors that the pores were pas- 

 sages for the protrusion of internal organs connected with the 

 vitality of the animal. The fact, however, that the pores do not 

 penetrate into the general cavity of the body, disproves this 

 theory ; and, moreover, through many of the tubercles — those 

 with a vesicular and spongy summit, such protrusion would be 

 utterly impossible. 



In Caryocrinus ornatus there are thirty hydrospires arranged 

 as follows : 



1. Ten at the base — half of each on a basal plate, and the 

 other half on one of the subradials, their longer diagonal vertical. 



2. A zone of six around the fossil at the mid-height — their 

 longer diagonals horizontal. These seem to be imperfectly 

 developed, for, on the inside, the tubes occupy only a small space 

 in the centre. 



3. A third band of fourteen — two of them with their longer 

 diagonals vertical, and the others arranged in six pairs, the 

 diagonals of each pair inclining toward each other, upward, at an 

 angle of about 30°. There are only three interradii in Caryo- 

 crinus : the mouth is placed in one of them, and the two hydro- 

 spires with vertical diagonals in the other two. 



In Pleurocystites the hydrospires are also of a rhomboidal form, 

 but instead of having the tubular structure of Caryocrinus, they 

 consist of a number of parallel inward folds of an exceedingly 

 thin part of the shell. These folds no doubt represent the tubes 

 of Caryocrinus. If we grind down a hydrospire of this latter, 

 so as to remove all the shell, and expose the edges of the tubes, it 

 then exhibits precisely the same form as fig. 5 a. i.e., the form of 

 a rhomb, longitudinally striated at right angles to the suture, 

 and with no pores. The tranverse section in Pleurocystites only 

 differs from that in Caryocrinus in having no shell between the 

 points c c. In the hydrospire of Pleurocystites rohustus, of the 

 Trenton limestone, we have the commencement of the formation 

 of an internal gill with a single spiracle. The surface is not flat, 

 as it is in many species, but concave, as shown in the section ; 

 and it is evident that if the concavity should be carried further, 

 and, at the same time, the points c c made to approach each other, 

 the effect would be to produce an elongated sack, deeply folded on 



