III. Lebetodiscus Dicksoni. 



1908 549 



(perhaps for the reception of smaller plates), but do not appear to be 

 of the same character as either the flooring-plates or the covering- 

 plates in Bigsby's specimen of A. Dicksoni. It is, in fact, clear 

 that the specimen figured by Dr. Jaekel differs in its peripheral 

 zone and in its subvective skeleton from our species, and that in 

 those points it has the character of an ordinary Agelacrinus. 



With the language used by Dr. Jaekel it is not easy to reconcile the 

 following sentences in Dr. Clarke's paper (op. cit., p. 191): — " Billings 

 claimed that in the Trenton species A. dicksoni, perforated ambulacral 

 plates were exposed, but this observation has not been confirmed and 

 Jaekel holds that no ambulacral plates were present in these bodies. 

 At all events usually only the cover plates have been observed." 

 I am not sure what this means, but it is certain that the plates 

 between which Billings described ' indentations ' were those here 

 called flooring-plates, and it is highly probable that these indentations 

 were podial pores. It is also certain that these same plates were 

 identified by both Billings and Jaekel with the ' covering-plates ' or 

 4 ambulacrals ' of a crinoid arm, and that they did not mention the 

 smaller covering-plates, which, in my opinion, are the only homologues 

 of crinoid ' Saumplattchen.' 



Of recent years the only other reference to the species has been the 

 record of its occurrence in Trenton Limestone at Pakenham, Ontario, 

 by Dr. H. M. Ami (Ann. Rep. Geol. Surv. Canada, xiv, p. 84 J ; 

 January, 1905). 



Systematic Relations of the Species. 



There still are problems to solve with regard to Agelacrinites 

 Dicksoni ; but there are problems presented by the majority of 

 Edrioasteroidea, and we certainly know enough to make comparison 

 with other forms profitable. 



Taking the families of Edrioasteroidea as defined in Lankester's 

 " Treatise on Zoology " (vol. iii, pp. 207-9, 1900), we may at once set 

 aside the Cyathocystidae with their massive theca, and the Stegano- 

 blastidse with their stem. 



Turning to the Agelacrinidse, with which the species has always 

 been placed, we see that from Stromatocystis it is separated by the 

 imbrication of the iuterradial plates and the curvature of the rays. 

 The latter feature also distinguishes it from Cystader and Hemicystis. 

 It is further separated from Cystaster by the large size of its inter- 

 radials, and from Hemicystis, A gelacrinus, Streptaster, and Lepidodiscus 

 by the absence of the differentiated marginal zone, which in those 

 forms is always obvious and often highly differentiated. I also 

 incline to regard it as having had a less flattened and less sessile habit 

 than the genera just mentioned. Similar features, as well as the 

 clear alternation of the flooring-plates of the grooves, enable us to 

 discriminate between it and the little-known Ilaplocystis of Roemer. 

 As for the Carboniferous form to which in 1897 Gregory gave the 

 name Discocystis, we know, at all events, that it had no imbricating 

 plates, and that the margin was more distinct than in A. Dicksoni. 



A more important character than any of those mentioned is 

 presented by the structure of the subvective skeleton. It seems 



