182 THE VOYAGE OF H.M.S. CHALLENGER. 
anambulacral plates of recent Crinoids (Pl. XVII. fig. 6; Pl. LV.). They pass down- 
wards into the interradials at the sides of the’calyx, just as in the recent species and in 
the Liassic Hatracrinus. 
Wachsmuth stated in 1877' that although he had not found the summit of any 
Ichthyocrinoid perfectly preserved, he felt convinced from what he had observed “ that it 
did not consist of a soft skin.” Subsequently, however, he described the ventral disk of 
the Ichthyocrinid” as “rarely preserved; composed of a more or less soft or scaly 
integument, yielding to motion in the body and arms. . . . The interradial areas are 
sometimes found depressed and im other cases distended, showing that there had been 
some expansion or contraction of the body-walls due to the mobility of the radial parts, 
and indicating likewise flexibility in the vault.” 
Under these circumstances I find it difficult to believe that the ventral disk of the 
Ichthyocrinidz did not correspond to the similarly named structure in recent Crinoids, 
but represents the solid vault of Actinocrinus. Were this the case, there must have been 
another flexible skin inside the “ pliant scaly integument,” with the food-grooves passing 
over its upper surface as they do over that of an internal cast of Actinocrinus. It is of 
course impossible that a proof of the existence of such a structure can ever be obtained. 
But why should its existence be postulated at all, simply because Ichthyocrinus is a 
Paleeocrinoid 2 
According to Wachsmuth and Springer “this family might very appropriately be 
called the Articulates of the Paleozoic Crinoids, being especially distinguished in most of 
its species by what seems to be an articulate structure in the whole skeleton.”* I cannot 
but believe that they present a similar approximation to Neocrinoids in the structure of 
their vault, ventral disk, or whatever else it be called. 
Any Crinoid with a well-plated disk (Pl. XIII. fig. 1; Pl. XVII. fig. 6; Pl. XXVI. 
figs. 1,2; Pl. L. figs. 1, 2; Pl. LV.) appears to me to be a recent Onychocrinus. If the 
summit of this genus was soft, pliant, and flexible, it must have consisted like the ventral 
disk of a Pentacrinus of a perisome formed of connective tissue, with the numerous 
irregular interradial plates imbedded in it ; and I cannot bring myself to believe that the 
flexible summit was really the “tegmen” overlying another disk, which itself represented 
the plated ventral perisome of Pentacrinus. It is admittedly a direct continuation of 
the “squamous integument” uniting the rays on the dorsal side. ‘This is ‘“‘ composed of 
very minute irregular polygonal plates, or by distinct interradial and axillary plates, the 
former varying in number from one to thirty or more.* 
Thus Onychocrinus has a large first interradial which rests between the first and 
second radials. “‘ The succeeding ones are smaller, decrease rapidly in size and thickness, 
and pass gradually into the very minute irreeular plates which form the interradial 
1 Amer. Journ. Sev. and Arts, vol. xiv. p. 185 2 Revision, parti. p. 31. 
3 Tbid., part i. p. 31. 4 Ilid., part i. p. 31. 
