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



(if the body, they are often counected by a pavement of minute irregular plates, which 

 may commence as low down as the level of the second radials, and are thought Ijy 

 Wachsmuth and Spriuger to indicate the presence of a flexible perisome. This is 

 especially the case in some species of Taxocrinus, the rays of which must have been at 

 least as free as those of many Pentacrinidse and Comatulse, and much more so than those 

 of Apiocrinus and Guettardicrinus. In these two genera, as in Uintacrinus, the calyx 

 interradials are not only present but well developed, so as to increase the size and com- 

 plexity of the cup. In fact the rays of Guettardicrinus are immovably united as far as 

 the second brachial, either directly, or by the intervention of interradial plates ; whUe 

 some species of Apiocrinus (Ajnocrinus -parhinsoni) have the second and third radials in 

 close lateral contact with their fellows. Other species, however, with the arms free from 

 the radial axUlaries, have a well defined pavement of interradial plates, the lowest of which 

 are large and regular and rest on the upper angles of the first radials, as in Ap>ioc7'inus 

 roissyanus ; ' while the upper ones are smaller and more irregular, and pass gradually 

 upwards into those of the ventral side. The same is the case in Marsupites. 



Many Pentacrinidse and Comatiilse have wide rays which are in close lateral contact 

 just as in Ajnocrinus parhinsoni (PL XV. fig. 2 ; Pis. XVIII., XIX.; PI. XXV.; 

 Pis. XXVIII.-XXX.), while others have the rays more separated from one another, but 

 united by flexible perisome in which the joints of the lower pinnules and numerous small 

 irregular plates are imbedded (PL XIII. fig. 1 ; PL XXXI. ; PL XXXVII. fig. 1 ; 

 PL XXXIX. fig. 1 ; PL XLIX. figs. 1,2; PL L. fig. 1). These may cease at the level 

 of the thii'd axillaries, or pass up into the plating of the ventral side as in Apiocrinus 

 roissyanus, Marsupites, and the Liassic species oi Extracrimis. But they are never so large 

 as in these fossils, and more nearly resemble the small ii-regular plating of the Ichthyocrinida3. 



Thus then there are many Neocrinoids with no interradial plates in the calyx ; and 

 when these plates are present and well defined, as in Apiocrinus, Guettardicrinus, and 

 Marsupites, or Uintacrinus, they are not limited to any special side of the calyx, but 

 are equally distributed all round it ; so that there is no distinction of the anal side, 

 Thaumatocrinus of course excepted. 



In the Palasocrinoids, however, the pentamerous symmetry of the calyx is almost 

 always disturbed by a greater or less modification of the plates on the anal side. The 

 difl'erence may be very slight, as in Phimocrinus and Cupressocrinus, which have the 

 anal opening separating the muscle-plates of two adjacent radials. But even this 

 character apjiears to be absent in the remarkable genus Erisocrinus from the Upj)er 

 Carboniferous of America, which has a calyx unusually Uke that of Encrimts; and also in 

 Steminatocrinus from the Eussian Carboniferous, which is still more like Encrinus in the 

 structure of the arms. 



Some forms have a special anal plate between two of the primary radials. This is the 



1 See woodcut, fig. 9 on p. 183. 



