234 Mr. F. A. Bather on British Fossil Crinoids : 



the arms, but apparently does not extend to their full length. 

 Arms uniserial, very long, tapering little, bifurcating at 

 lengthening intervals toward the upper parts into very nume- 

 rous equal branches, the ultimate divisions being extremely 

 attenuate." " Arm-joints shorter than in Crotalocrinus^ vvilli 

 parallel sutures ; those of adjacent branches opposite each 

 other not alternating." " Ambulacral furrows shallow, with 

 covering-plates arranged in the usual way." " Column 

 round, very large, with short joints and thin walls ; canal 

 round and of extremely large size." 



It is not, of .course, any one of these points of resemblance 

 that is remarkable ; it is the total effect : the evidence, so to 

 speak, is cumulative. But there are two points in the struc- 

 ture of the Crotalocrinidaj on which Wachsmuth and Springer 

 have laid particular stress. 



The reticulate structure of the arms in Crotalocrinus 

 depends on the combination of the following characters : — 

 depth of ossicles dorso-ventrally, length of arms, extreme 

 bifurcation at regular intervals, and lateral processes of ossicles. 

 In all these points the arms of Enallocrinus resemble those of 

 Crotalocrinus except that the bifurcation does not take place 

 at such regular intervals, and the arms are not laterally con- 

 nected. A development in the direction of such connexion 

 has, however, been demonstrated by Wachsmuth and 

 Springer. " Toward the upper ends of the arm-joints there 

 are more or less conspicuous transverse projections — one from 

 each side of the joint — which are more prominent and elon- 

 gate at the ventral side. They border the arm-furrow, and 

 give to the arm, when viewed from the side, a pectinate 

 appearance, which is more strongly marked toward the distal 

 ends of the arms." " We have observed these projections on 

 the arms," add Messrs Wachsmuth and Springer, " only in 

 the English specimens. We give it as a generic character, 

 as we think it likely the Swedish ones will show it also when 

 sufficiently well preserved ; and because we consider it of 

 some importance, as representing the projections on the arms 

 of Crotalocrinus by which these were connected, and thus 

 exhibiting a tendency toward the reticulate structure." Now 

 the arms of Thenarocrinus not only resemble those of Enallo- 

 crinuSj and to a less extent of Crotalocrinus^ in depth, length, 

 and bifurcation, but they show undoubted indications of 

 nascent lateral processes. In their position on the arm, and 

 on the ossicle, and indeed in everything but size, the antero- 

 lateral ridges of Thenarocrinus resemble the processes of 

 Enallocrinus. 



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