260 Mr. E. Billiiiiifs on tlw Structure of 



( 'ori/ocruius ; the mouth is placed in one of them, and tlie two 

 hydrosj)ires with vei'tical diagonals in the othei* two. 



In I'/eurocf/fiti'tcs the hydrospires are also of a rhomLoidal 

 form; but, instead of iiaving the tubular structure of (.'(iri/o- 

 criuuSj they consist of a mimber of parallel inward folds of an 

 exceetlingly 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 witli no pores). The 

 transverse section in Phurocystites t»nly differs from that in 

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

 the hydrospire of Pleurocystites robusttis, of the Trenton 

 Limestone, we have the commencement of the formation of an 

 internal gill with a single spiracle. The surface is not tiat, as 

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

 and it is evident that if the concavity Averc 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, 

 dee})ly 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 Ave contracted the four sides, gradually curving them outAvard 

 at the same time, but not diminishing the superficial extent 

 of the Avails of the folds, although altering tlie form to corre- 

 S[)ond Avith the decreasing aperture, the result Avould be a 

 deeply folded flask-shaped sac, Avitli a small round orifice 

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



In Pakeocystites tenuirccdiatus, a species very characteristic 

 of the Chazy Limestone, the Avhole surface (in the condition in 

 Avhich the fossil is usually foimd) is coA'cred Avith deeply striated 

 rhombs, the fissures being dec})est Avhere they cross the suture, 

 and groAving gradually shalloAver as they approach the centre 

 of the plates, Avhere they die out altogether. Detached plates 

 occur in vast abundance, but no perfect specimens liaA^e ever 

 been found. I discovered, hoAvever, several fragments of the 

 body sufiHeient to give the general form and to show that, 

 Avhen the surface is perfect, all these fissures are comj)lctely 

 covered over by a A'cry thin shell, and that Avhen they cross 

 the suture, there is a small pore in the bottom of each Avhieh 

 penetrates to the interior. The rhombs of this species arc 

 thus external hydrospires. The fissures seen in the ordinary 

 Aveathered specimens are the remains of Hat tubes like those 

 of CaryocrinuSj situated on the outer instead of the iiuier 

 surface of the test. The ehylaqueous fluid passed outward 



