Tin; STEM \.\l> c \\.\ \. 27 



were generall) called bj this name, until Billings 1 asserted that thej cannot represent 

 the radials of a Crinoid because they do not support the bases of the ambulacra, 

 which "are situated in the apex of the fossil, and do not come into contact with the 

 forked plates. The apex of Pentremites is identical with the actinal centre of sea- 

 urchins and starfishes, in which the mouth is situated. It is here that the ambulacra 

 originate, and grow outward by the addition of new plates to their distal extremitii 8, 

 There can be little doubt that such was the mode of growth of the ambulacra of the 

 Pentremites. The smaller extremity, therefore, of their ambulacra, which is received 

 into the forked plate, is not the base, but corresponds with the apex of the ambu- 

 lacrum of a sea-urchin or of a starfish. It also represents the tip of the arm of a 

 Crinoid." 



The distal end of the ambulacrum of Pentremites undoubtedly corresponds to the 

 apex of the ambulacrum in an Urchin. But Billings also says that the bases of the 

 ambulacra do not come into contact with the fork-plates of Pentremites, implying at 

 the same time that the radials of a Crinoid do support the bases of the ambulacra. 

 It appears to us, however, that the relation of the fork-pieces to the proximal ends of 

 the ambulacra in Pe/itremitidea or Troostocrinus (PI. IV. figs. 13, 14, 17, 18; PI. V. 

 figs. 4, 6, 15, 21 ; PI. XII. fig. 11) is much closer than that between the radials of many 

 Crinoids and the peristome in which their ambulacra originate. A comparison of the 

 fork-pieces of Codaster trilobatus (PL XIII. figs. 3, 4), and of CryptoseJiisma Schulzi 

 (PL V. figs. 23-26), with the radials of Platycrinus, Diehocriims, or Hyocrinus shows 

 conclusively the identity of the two sets of plates. Each of them receives the distal 

 end of an ambulacrum which starts from the peristome ; and each alike may be said 

 to support the base of the ambulacrum. But while the ambulacrum of the Blastoid 

 goes no further than the radial, that of the Crinoid extends on to the arm-joints which 

 are successively added beyond the radial, so that the tip or apex of the ambulacrum 

 is carried further and further away from the calyx, to which it was at first confined. 



In many Blastoids, as in many Urchins, the development of the interradial areas, 

 whether due to the great size of the deltoid plates or to the length of the radial 

 limbs, causes a considerable distance to intervene between the actinostome and the 

 bodies of the radials which correspond to the ocular plates of the Urchins, so that 

 the primitive relation of the radials to the ambulacra becomes somewhat obscured 

 (PL IX. figs. 1-6). But the same is the case in many Paheocrinoids which have a 

 largely developed interradial system between the primary arm-divisions, so that the 

 cup increases considerably in size ; and we do not see how there can be any doubt 

 respecting the mutual homology of the fork-plates of a Blastoid, the radials of a 

 Crinoid, and the ocular plates of an Urchin. 



It may be noted further that the addition of an articular facet to the radial lip of 



i American Journ. Sci. 1870, vol. 1. p. 226; Ann. & Mag. Nat. Hist. 1871, vol. vii. p. 144. 



f. 2 



