22 CATALOGUE OF THE BLASTOIDEA. 



losed joint were more extended, and became almost entirely absorbed into tbe basal 

 plates, leaving only external marks, while at the inner ones the column structure can 

 yet be recognized, although the joint here has actually become a part of the basal 

 plates." The regular distribution of the clover leaves along the depressed lines of 

 the interbasal sutures which correspond to the great hollows in the base of Triccelo- 

 crinus (PI. XIX. fig. 14) is very marked, and is particularly well shown in one of our 

 specimens of Pentremitidea Paillettei. The lower part of the basal cup is covered by 

 a tolerably regular overgrowth, near the bottom of which a definite circular line is 

 visible, just as in Pentrcmitcs; but three lancet-shaped processes extend upwards 

 from this and occupy the hollows between the convexities of the basals. This over- 

 growth seems only to occur in those Blastoids which have relatively large and 

 symmetrical basals, and not perhaps in all of these ; for, as already remarked, we 

 have no knowledge of it in Troostocrinus, Triccelocriiius, and Stephanocrinus, nor in 

 Granatocrinus, Cryptoblastus, Scliizoblastus, and Mesoblastus, nor in the asymmetrical 

 Eleutherocrinus and Astrocrinw. The tripartite division of the basals is, however, 

 common to all Blastoids. 



D. The Radials. 



The fork-pieces or radials, which rest upon and alternate with the basals of a 

 Blastoid, present a great amount of variation in their characters. Homologous with 

 the radials of a Crinoid, and therefore also with the ocular plates of an Urchin, they 

 differ altogether from the former in bearing no articulated arms, while they closely 

 resemble the latter in their relation to the distal ends of the ambulacra, as is well 

 shown in Granatocrinus Derbiensis (PL IX. fig. 4), and still more markedly in Elcea- 

 crinus Verneuili (PL II. fig. 45). 



Several among the great variety of forms which are presented by the radials of the 

 Blastoids correspond to similar types among the Crinoids. Thus, for example, the more 

 or less spade-like radials ofCodaster trilobatus (Plate XIII. fig. 3) closely resemble those 

 of Dichocrinus, or of the recent Hyocrinus. In all three alike there is a somewhat 

 well defined medio-dorsal convexity which becomes more prominent towards the 

 ventral edge of the plate. But while in Hyocrinus this marks the articulating facet 

 of an arm on to which the ambulacrum passes from the disc, the ambulacrum of 

 Codastcr has no brachial extensions but terminates in the radial lip. The ventral 

 surface of the radial (PL XIII. figs. 1, 4) is also wider than that of the corresponding 

 plate in Hyocrinus, which is little more than an edge for the support of the first 

 brachials, and does not contribute in any way to form the roof of the visceral cavity. 

 In the Codastcr, however, the ventral surface of the radials is somewhat extended 

 right and left of the ambulacrum, and supports the distal ends of the lamellar 

 hydrospire-sacs which dip down some way into the visceral cavity (PL XIII. figs. 1, 4, 

 5-8). The general relation of the calyx of Codaster to its ambulacra finds a very 



