44 SMITHSONIAN INSTITUTION 



conspicuous in the Lecanocrinidae, where it rarely appears in the visible parts 

 of the rays, the sutures for the most part being perfectly straight. In the later 

 Sagenocrinidae, and in the majority of Ichthyocrinidae and Taxocrinidae, it 

 becomes a most prominent feature in their habitus. Forbesiocrinus, Ichthyo- 

 crinus, Taxocrinus and Onychocrinus are fine examples of the arcuate sutures 

 which continue down to the radials (Pis. XXVI, XXXIII, LV, LVIII, and 

 LXXII). This type of structure was clearly an accompaniment of the great 

 mobility in calyx and arms characteristic of the group. Many Recent pen- 

 tacrinites have similar arcuate suture lines at the brachial syzygies (Challen- 

 ger Report on the Stalked Crinoids, pi. 29, fig. 2). 



When extremely developed, the projecting processes sometimes tend to 

 become fractured, perhaps from contraction at death or during fossilization, 

 thus giving the appearance of separate intercalated plates (PI. XXVI, figs. 3b, 

 4, 5). This feature has given rise to the idea of " patelloid plates," which has 

 figured to a considerable extent in the literature from Hall, who originated it, 1 

 to Bather, 2 who treated and figured these apparent plates as a frequent charac- 

 ter of the Flexibilia Impinnata. The superficial appearance of many specimens 

 certainly gave good ground for such an interpretation. But by means of 

 entirely isolated plates (Pis. XXIV, figs. ga, b, lya, b; XXVI, figs, lb, c) ; of 

 weathered specimens in which the sinuous sutures are eroded on the exposed 

 parts while present and distinct in other parts of the same specimen (PI. LVII, 

 fig. 9) ; and of grinding, which causes the curves as well as the apparent lines 

 of articulation of the processes to disappear at a very slight depth below the 

 surface (PI. LXXII, fig. 10) ; — I have been able to ascertain the actual struc- 

 ture of these parts. This I have illustrated by the accompanying text-figures : 

 6a to d, made from a pair of isolated plates, giving dorsal and ventral views; 

 and Ja to c, cross-sections of three plates in progressive stages of development 

 of the processes, together with the articulating face, d, of the most advanced 

 of these three. 



From these, as well as from numerous figures on plates above referred to, 

 it will be seen that the projections and their sockets, and the apparent curving 

 of the suture, are almost superficial structures, and that at a short distance 

 inward, as well as at the ventral side of the plates, the sutures are perfectly 

 straight. 



To explain the mechanics of the " patelloid " structure, it may be described 

 as consisting of a thin process projecting downward from the middle of the 

 curved proximal margin of the brachials, and occupying a socket of correspond- 

 ing shape formed for its reception on the distal margin of the apposed brachial. 

 The outer convex surface of the process conforms to the general curvature of 



1 Geology of Iowa, vol. I, pt. 2, 1858, p. 630. 



" In Lankester, Treatise on Zoology, pt. 3, 1900, p. 190. 



