Mr. F. A. Bather on British Fossil Grinoids. 379 



development of these genera has proceeded be prolonged back- 

 wards instead of forwards, we arrive at an ancestral image 

 not unlike Ectenocrinus, Heterocrinus, or Iocrinus. We 

 therefore conclude that these latter forms approach more 

 nearly to the common ancestor. It remains to be seen which 

 of the three is the most ancestral. In the first place Iocrinus 

 has a simpler structure so far as the radials proper are con- 

 cerned ; and that in this case the simplicity is archaic I have 

 already tried to show (p. 327, end of first paragraph). 

 Secondly as regards the anal plates I have argued that 

 Iocrinus represents a more primitive condition (top of p. 330). 

 Thirdly the arms are simply dichotomous and not at all 

 specialized into main arm and armlets. These arguments 

 should be enough ; but to my mind the existence of an exactly 

 similar and contemporaneous form among the Dicyclica — viz. 

 Merocrinus — proves that the two were descended from an 

 ancestor possessing their common characters, and probably 

 witli a Dicyclic base. This ancestor then, if its existence be 

 granted, was likewise the progenitor of Ectenocrinus and 

 Heterocrinus, and, among the Dicyclica, of Ottawacrinus and 

 probably Carabocrinus, and possibly of Dendrocrinus and 

 Euspirocrinus, 



The Monocyclic base is of course enough to separate Ecteno- 

 crinus , Iocrinus, &c. from the Dicyclic genera, and at their 

 subsequent history we shall now do well to glance. Whether 

 Heterocrinus or Ectenocrinus be the older is hard to say • 

 Heterocrinus probably, as the brachianal is in a slightly more 

 primitive position ; the arms also are simpler in that the arm- 

 lets are not so reduced in size, and the syzygial union of 

 alternate joints characteristic of Ectenocrinus is not here deve- 

 loped. Be this as it may, Ohiocrinus is, both in time and 

 arm -development, a natural descendant of Heterocrinus. 

 Anomalocrinus is a peculiar offshoot from the same stock. 

 Further than this it is impossible to trace the history of this 

 family. Edriocrinus, Belemnocrinus, and Holocrinus, though 

 Monocyclic, had certainly a different origin ; Edriocrinus, 

 however, is so extremely specialized in other respects that one 

 cannot perceive its true affinities. 



Mycocrinus and Catillocrinus appear to be connected in 

 the structure of the dorsal cup with Calceocrinus ; but the 

 drooping on the stem, which seems to have been the main 

 factor in inducing the structure of Calceocrinus , does not 

 operate here. The resemblance may be merely homoplastic j 

 if anything more, we must suppose that structures originally 

 selected as conducing to one special object, were on a sudden 

 diverted to another quite distinct. The peculiar arm-arrange- 



27* 



