1869.] BILLINGS — STRUCTURE OF CRINOIDS, ETC. 287 



one side, and with a fissure extending the whole length on the 

 other side. The transverse section of such a sack would be fig. 

 6, the same as in Pentremites. Again, if we contract the four 

 sides, gradually curving them outward at the same time, but not 

 diminishing the superficial extent of the walls of the folds, 

 although altering the form to correspond with the decreasing 

 aperture, the result would be a deeply folded, flask-shaped sack, 

 with a small round orifice like fig. 7, which is the internal gill of 

 a spider. 



In Paloeocystltes tenuiracUafus, a species very characteristic of 

 the Chazy limestone, the whole surface (in the condition in which 

 the fossil is usually found) is covered with deeply striated rhombs, 

 the fissures being deepest where they cross the suture, and grow- 

 ing gradually shallower as they approach the centre of the plates, 

 where they die out altogether. Detached plates occur in vast 

 abundance, but no perfect specimens have ever been found. I 

 discovered, however, several fragments of the body sufficient to 

 give the general form, and to show that, when the surface is 

 perfect, all these fissures are completely covered over by a very 

 thin shell, and that, when they cross the suture, there is a small 

 pore in the bottom of each, which penetrates to the interior. 

 The rhombs of this species are thus external hydrospires. The 

 fissures seen in the ordinary weathered specimens are the remains 

 of flat tubes, like those of Cari/ocrlnus, situated on the outer 

 instead of the inner surface of the test. The chylaqueous fluid 

 passed outward through the pores and filled the tubes, to be 

 aerated through the thin external covering by the surrounding 

 water. In Gart/ocrinus the water passed inward, through the 

 pores, into the tubes, and aerated the fluid within the general 

 cavity of the body. 



The discovery that the fissures and pores of the Cystidea, do 

 not communicate directly with the general cavity of the body is 

 entirely due to Mr. Rofe. After reading his highly important 

 paper, I re-examined a great number of specimens, and found 

 sufficient to confirm his observations. 



3. On the genus Cadaster. 



Every author who has described a species of this genus, has 

 remarked the peculiar striated areas in the interradial spaces. 

 Prof. McCoy, the founder of the genus, pointed out their resem- 

 blance to the hydrospires of the Cystidea; but it was Mr. Rofe 



