294 Messrs. P. H. Carpenter and R. Etheridge, Jun., 



and a good deal further in many types ; while sutural union 

 in a vertical direction is not exclusively confined to the Palse- 

 ozoic Crinoids, though it is certainly more common among 

 them than among the younger forms. But at the same time 

 there were several genera (and those by no means the least 

 common) in which the second radials were not only free from 

 their fellows, but articulated to the first radials, in precisely 

 the same manner as the corresponding joints of a Pentacrinus 

 or Comatula. 



Other genera, however, are characterized by a peculiarity 

 which is only met with among the Paljeocrinoidea, viz. the 

 absence of any distinct articular surface on the distal faces of 

 the first radials, which are not perforated by canals for the 

 axial cords of the rays. The presence of these canals is 

 mentioned by Zittel among the characters of the Mesozolc and 

 younger Crinoids ; but their absence is not distinctive of the 

 Palagocrinoids, as they exist in Platycrinus and in all the 

 forms with true articular facets on the first radials. 



There are several Palaeozoic types, however, in which the 

 second radials were in contact with the first by semicircular 

 or horseslioe-shaped surfaces, with or without notches for the 

 reception of the axial cords at the bottom of the concavity. 

 This, though a permanent condition in some Palgeocrlnoids, 

 is a transitory one In the young Comatula^ and, as seen above, 

 in the young Allagecrinus ] and as none of the Neocrinoidea 

 (if we may so call them), with the doubtful exception of 

 Comastevj Goldfuss *, retains this peculiarity when mature, it 

 is, as far as it goes, a good general character for separating 

 the younger from the older Crinoids. 



Another and a better distinction between them is one on 

 which considerable stress has been recently laid by Messrs. 

 Wachsmuth and Springer t, who believe that the mouth was 

 internal in most Palseozoic Crinoids, if not in all of them ; 

 while it is external and suprategminal in the recent forms, for 

 which they propose the general term Stomatocrinoidea. 



That the mouth was internal in the Actinocrlnldse we have, 

 of course, not the smallest doubt ; but we would point out 

 that the " vault " of this family, closing in the mouth and all 

 the covered ambulacra of the body, is a very different struc- 

 ture from the six " apical dome-plates " of the Cyathocrinldse 

 and Ichthyocrinidge, which merely close the peristome and 

 leave the plated ambulacra as much external as those of any 



* Linnean Society's Journal, Zoology, vol. xiii. pp. 454-4o6. 



t Op. cit. pp. 6, 30. See also " Notes on the Internal and External 

 Structure of Palaeozoic Crinoids," by Charles Wachsmuth ('American 

 Journal of Science and Arts,' 1877, vol. xiv, pp. 117-127 and 181-190). 



