260 Mr. E. Billings on the Structure of 



Caryocrinus] the mouth is placed in one of them, and the two 

 hjdrospires with vertical diagonals in the other two. 



In Fleurocystites the hydrospires are also of a rhomhoidal 

 form ] but, instead of having the tubular structure of Caryo- 

 crinus, 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 presents precisely the 

 same form as tig. 5 a {{. e. the form of a rhomb longitudinally 

 striated at right angles to the suture, and with no pores) . The 

 transverse section in Fleurocystites only differs from that in 

 Caryocrinus in having no shell between the points c c. In 

 the hycb'ospire of Fleurocystites robustus, 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 were carried further, 

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

 other, the effect would be to produce an elongated sac, 

 deeply folded on one side, and with a fissure extending the 

 whole length on the other side. The transverse section of 

 such a sac would be fig. 6, the same as in Pentremites. Again, 

 if we contracted the fom' 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 corre- 

 spond with the decreasing aperture, the result would be a 

 deeply folded fiask-shaped sac, with a small round orifice 

 like fig. 7, which is the internal gill of a spider. 



In Fala-ocystites tenuiradiatus, 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 growing gradually shallower as they approach the centre 

 of the ])lates, 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 Caryocrinus^ situated on the outer instead of the inner 

 surface of the test. The chylaqueous fluid passed outward 



