556 p. H. CAEPENTER ON CRETACEOUS COMATVLM. 



column was in some cases, and perhaps more or less in the Palaeo- 

 crinoids generally, subservient to respiration." According to these 

 authors, some of the Palseocrinoids had hydrospires like those of 

 the Blastoids ; but there are a large number of forms in which no 

 trace of them has as yet been discovered. They suggest therefore 

 that the complex stem which many of the Palseocrinoids possess 

 " was a means of communication between the water outside and the 

 internal organs of the body for some purpose," probably respiratory. 

 In some groups, such as the Platycrinidse, the stem is destitute of 

 any internal structure, but is simply pierced by a very small central 

 canal, which lodged the vascular axis. This canal is of very small 

 size in Pentacrinus ; but it is sufficient to contain six vessels, a 

 central one and five peripheral ones, which in the nodal joints send 

 branches into the cirrhi. In the fossil Pentacrini the central canal 

 of the stem is very small, just as in the recent species. It is also 

 small in Encrinus, and its upper end is enclosed by the circlet of 

 five under basals, each of which bears a slight pit on its ventral 

 surface. These pits, which are radial in position, are the dorsal ends 

 of the furrows that descend the sloping ventral faces of the radials. 

 At the margin of the calyx they are continuous with the more or 

 less distinct grooves along the ventral surface of the skeleton of the 

 rays and arms, just as in Comatula and Pentacrinus. The radial pits 

 of Encrinus are thus precisely homologous with those on the upper 

 surface of the centrodorsal in Comatula. If, instead of being pits, 

 they were perforations through the under basals, the lower surface 

 of the calyx would show five large apertures surrounding a central 

 one, instead of that central one only. 



This condition, or a parallel one, is precisely what we do find in 

 some fossil Crinoids. In Nannocrinus and Myrtillocrinus * the 

 body is pentamerous, but the stem only tetramerous; and there 

 are five apertures on the under surface of the calyx. Judged by the 

 standard of Pentacrinus and Phizocrinus, the central one is more than 

 large enough to have lodged the vascular axis ; while there are four 

 peripheral ones which lodged, I believe, downward prolongations of 

 the coelom. In one species of Epactocrinus (E. antiquus) the base 

 of the calyx has five separate apertures, while in another (E. irregu- 

 laris) there is but one large cruciform opening taking up nearly the 

 whole of the top stem-joint. This looks as if the central aperture 

 had fused with the four peripheral ones. The same must have been 

 the case in Cupressocrinus, in which the under surface of the calyx 

 shows a four- or five-rayed perforation, while the stem-joints have 

 four or five separate openings around a large central one." This 

 is relatively far larger than that of a Pentacrinus-stem, and is 

 sometimes four- or five-lobed, as if corresponding to the four or five 

 peripheral vessels of the vascular axis. What, then, could have been 

 lodged in the peripheral canals of the stem but tubular exten- 

 sions of the body-cavity homologous with those which end on the 

 centrodorsal piece of most Comatulce? In Ant. perforata, and 



* See Queustedt's ' Petrefactenkunde Deufcschlands,' Bd. iy. " Asteriden und 

 Encriniden," tab. 108. 



