MORPHOLOGY 25 



ever, such as Ichthyocrinus, Forbesiocrinus, and Parichthyocrinus , the walls 

 are so thick that the fossils are often perfectly rotund. This feature of ex- 

 treme thickness of the plates may be seen in various figures on Plates XXI, 

 XXIII, XXIV, XXXI, XXXIX, LVII, LXI. I know of no instance of a 

 Flexible crinoid with thin calyx walls, except rarely at the very lowest part 

 in some forms of Ichthyocrinus (PI. XXXVI, fig. 6c). 



Another fact conducing to preservation was the tendency in the species 

 of many genera to close infolding of the arms at death, so that the entire crown 

 was rounded like a ball. This was aided by a peculiarity in structure which 

 extends throughout the entire group, and forms one of its most distinctive 

 characters, viz., that the arms, instead of being free and completely differen- 

 tiated from the calyx, as in the Inadunata and most of the Camerata, either 

 lie in close contact, or for a part of their length are connected by plates or 

 perisome, or form a direct continuation of the radial series of the calyx, so 

 that there is no distinct line of demarkation between them. This mode of con- 

 struction gives to these forms a peculiar habitus which is in marked contrast 

 with those of the other two orders, and by which the specimens can as a rule 

 be readily recognized from small fragments ; and from the strong predomi- 

 nance of this character it also results that distinctions between the subordinate 

 divisions of the group are not so sharply marked as are most of them in those 

 orders. 



Neither is the rarity of these fossils due to restricted distribution. They 

 occur in most of the crinoid-producing regions of America, Britain, and con- 

 tinental Europe, from Spain to Sweden and Russia. And they range from the 

 Lower Ordovician to the Lower Coal Measures (Pennsylvanian) ; though what 

 became of them in certain horizons and localities otherwise prolific in crinoids 

 is a mystery. In the Upper Ordovician of the Cincinnati area, and in the 

 Onondaga and Hamilton of the Louisville area, both of which have yielded 

 great numbers of other crinoids, not a vestige of any species of the Flexibilia 

 has ever been seen, although in the Hamilton of other localities they are not 

 unknown. 



Withal, the Flexibilia form a good zoological group, well balanced and 

 differentiated from others, containing within itself subordinate modifications 

 due to morphological changes of fully as great importance as those found in 

 other orders, and in some respects parallel to them. 



In the Monograph of the North American Crinoidea Camerata by 

 Wachsmuth and Springer, 1897, pp. 38 et seq., it was shown that the elements 

 of the crinoid skeleton fall into two groups, viz., primary plates, and secondary 

 or supplementary plates — the former being those first developed in the larva, 

 represented in every group of the class, and undergoing few modifications in 

 geological time, the latter being interposed between the primary plates and 



