182 THE VOYAGE OF H.M.S. CHALLENGER. 



anambulacral plates of recent Crinoids (PI. XVII. fig. 6 ; PI. LV.). They pass down- 

 wards into the interradials at the sides of the calyx, just as in the recent species and in 

 the Liassic Extracrinus. 



Wachsmuth stated in 1877^ that although he had not found the summit of any 

 Ichthyocrinoid jserfectly 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 Ichthyocrinidaj " as " rarely presei'ved ; 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 in other cases distended, showing that there had been 

 some expansion or contraction of the body-walls due to the moliility of the radial parts, 

 and indicating likewise flexibility in the vault." 



Under these circumstances I find it difiicult to believe that the ventral disk of the 

 Ichthyocrinidse 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 

 Palseocrinoid ? 



According to Wachsmuth and Springer " this fjimily might very appropriately be 

 called the Articulates of the Palaeozoic 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 (PI. XIII. fig. 1 ; PI. XVII. fig. 6 ; PI. XXVI. 

 figs. 1, 2 ; PI. 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 irregular plates which form the interradial 



' Amir. Jmim. Sci. and Arts, vol. xiv. p. 185 ^ Revision, part i. p. 31. 



^ Ibid., part i. p. 31. * Ibid., part i. p. 31. 



