50 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



Carpenter ^ further states that in Pentacrinus Wyville-thomsoni the nodal 

 joint from which the separation takes place " sometimes loses its ordinary 

 characters altogether, becoming much enlarged and rounded below so as to be 

 almost hemispherical in appearance." And in a foot-note he says : " The un- 

 usual enlargement of the nodal joint suggests the idea that the structures 

 which have been described by Hall under the name Ancyrocrinus t may be the 

 detached stems of a Palseocrinoid in the semi-free condition/' — in which we 

 fully agree with him. Ancyrocrimis has the form of an anchor, with four hook- 

 like processes around a central ascending stem, which at its distal end is 

 provided with a small, rounded tubercle, closing the opening of the central 

 canal exactly as in some cases of Pentacrinus. That this stem is morphologi- 

 cally in the same condition as that of Pentacrinus, nobody will deny after 

 examining the specimens. The four lateral extensions were doubtless radi- 

 cular cirri, whose joints were obliterated by calcareous overgrowth, as in the 

 lower part of its tetramerous stem. 



Something similar to this may have taken place in other Palaeozoic Cri- 

 noids; and it is quite probable that the terminal end, as it appears in the 

 specimens, is in many cases not homologous with the part by which the 

 young Crinoid had been formerly attached, but is a product of later growth. 

 We suspect this to be the case in the Actinocrinidse, Platycrinidse, and other 

 forms in which the terminal part tapers rapidly to a point, and cirri are 

 given off from the sides. It would not be a great departure from the 

 structure of the Comatulae, if we bear in mind that their centro-dorsal is 

 a modified stem joint, bearing cirri upon its outer surface. The only 

 essential difference would be that in the Palseocrinoid the stem separated 

 at its lower end, and in the Comatulse at the uppero 



That the young Palaeocrinoid in its early life was attached by a dorso- 

 central, we may fairly infer from what we know of the development of the 

 Comatulae, and from palj^ontological evidence. The indications, however, 

 leave it somewhat doubtful if the fixation was permanent. We believe that 

 in the majority of cases among the older Crinoids the stem was afterwards 

 separated from the root, and that the animal subsequently led a free 

 life. In only two instances do we know that Palaeozoic Crinoids were 

 attached by what appears to have been originally a dorso-central plate : in 



* Challenger Rep. on the Stalked Crinoids, p. 19, and foot-note. 



t Eifteenth Ann. Rep., N. Y. State Cab. Nat. Hist., 1862, pp. 89, 90. 



