SAGENOCRINIDAE 235 



strata at Fern Glen, Missouri (fig. 9), and at Lake Valley, New Mexico (PL XXIV, 

 figs. 28,29). 



Unfortunately at none of these localities have we been able to recover any part of the 

 calyx intact; but from a locality in northern Ohio of corresponding stratigraphic position we 

 have a finely preserved species, the Forbesiocrinus communis of Hall, from the base of the 

 Waverly at Richfield (PL XX, figs. 4-8). This is thoroughly distinct from the fragments 

 at other American localities as to surface ornament, and from the Belgian species in other 

 ways, showing a considerable modification of characters, especially in the smaller inter- 

 brachial system and the distinct extension of a series of anal plates to the probable limit of 

 the tegmen. It is possible that if we had specimens of the Belgian form better preserved 

 in the original matrix we might find a slight extension of the anal series to the right, but it 

 could not have gone far, as there are no angles for sutural attachment at the margin of the 

 ray in the specimens we have. We should, however, expect individual variation in this 

 respect. 



Taking the two species together, it is easy to see that the geologically antecedent form 

 to which the genus is nearest related is Temnocrinus, of which it may well be a direct 

 successor. The development would be the same as from Protaxocrinus to Taxocrinus, pro- 

 ducing an equalization of the rays to three primibrachs all around. With this one change, 

 the two forms would fall readily under the same generic definition. 



Returning now to the first of the questions stated for discussion at the outset — the rela- 

 tions of this genus with Taxocrinus — it may be said here briefly that the two genera represent 

 the two diverging lines in the development of the anal structures of the Flexibilia, and are 

 therefore fundamentally distinct : Forbesiocrinus the stronger type of structure, in which 

 the posterior basal is followed by plates united to it and to adjacent rays by suture ; Taxo- 

 crinus the weaker, in which the posterior basal supports an anal tube bordered on one or both 

 sides by perisome. In the earlier type represented by the Belgian species, with the presence 

 of an anal series probably passing into a tube not originating on the posterior basal but at a 

 variable height above it, we have a stage somewhat intermediate between the extremes of 

 the families Sagenocrinidae and Taxocrinidae. With the wide divergence of its free rays, 

 not bounded by solid plates except at the lower part, it would represent in the former family 

 the tendency that Onychocrinus does in the latter. 



In the later type, containing the most numerous and prolific species, the genus repre- 

 sents the culmination of its family not only in time but in structure, as Taxocrinus and 

 Onychocrinus do in theirs. Here the calyx wall is extended by the growth of solid plates in 

 the anal and interbrachial areas until they are completely filled, and these plates merge gradu- 

 ally into the tegmen ; and in this respect we have such a complete contrast with the last 

 mentioned genera as the typical representatives of their family, that a more detailed 

 discussion of their respective characters will be instructive. It is the more so on account of 

 the confusion long existing between this genus and Taxocrinus, and the frequent shifting of 

 species from one to the other; the comparison will therefore be made chiefly with Taxo- 

 crinus, with appropriate reference to Onychocrinus in cases where the identical structures are 

 better represented. 



The fundamental distinction between the two types, as already indicated, is to be found 

 in the construction of the posterior basal and in the anal area, and as to the later species in 

 the interbrachial areas : 



1. The posterior basal and anal area. In Forbesiocrinus the posterior basal is elongated 

 and supports a succession of plates of more than one series which form a solid part of the 

 calyx wall; it is of uniform thickness throughout, is angular or truncate above, and is con- 

 nected with succeeding plates to the full thickness of its distal face by the same kind of 

 articulation or union as it is to the other adjacent plates. Except for its usually much larger 



16 



