24O SMITHSONIAN INSTITUTION 



clearly visible, proving it a part of the structure. Had it been otherwise, this thin projecting plate, so far 

 overlapping the plate below, would have prevented all motion in a direction outwards ; and since this 

 structure extended into the arms, we should expect to find there the means of free movement among the 

 plates. 



This description is fairly correct as to the facts ; the only fault to be found is with their 

 interpretation. That the indentation does not extend through the plate is important, and is 

 clearly proved by the form of the suture at the ventral side (PI. XXIV, fig. lyb) where 

 it is a straight line without any trace of sinuosity. But, instead of this projecting process 

 being " separately articulated and sometimes anchylosed," the fact is just the reverse. The 

 projection is a mere extension of the dorsal side of the upper plate from its proximal face, 

 thin, fragile, and tapering from its origin to a still thinner outer edge ; and it is sometimes 

 broken transversely from strain at death or during fossilization, so as to give the appearance 

 of a separate plate. This has already been fully explained in the chapter on Morphology. 



Hall's idea of " patelloid plates " was accepted by subsequent authors generally, includ- 

 ing Wachsmuth and Springer (Rev. Pal., pt. 1, p. 51)., and Bather (Lankester's Zoology, 

 pt. 3, p. 190). Miller and Gurley dissented (Bull. 3, Illinois St. Mus., 1894, p. 47), saying 

 that " the tooth-like projection, in Forbcsiocrimts agassisi, is not a separate plate; there are 

 no patelloid plates in any species of Forbesiocrinns" — a correct observation for which they 

 are entitled to due credit. 



Such " patelloid " processes, or arcuate sutures, are not confined to Forbesiocrinus, but 

 are widely prevalent throughout the whole group, and are especially strong in Taxocrinus, 

 from specimens of which we have interesting illustrations of their structure. Plate LVII, 

 especially figure 9, shows the presence of processes and arcuate sutures in some parts along 

 with perfectly straight sutures in other parts of the same specimen, where slightly worn by 

 weathering. 



The stem of Forbesiocrinus is of the proximally enlarging type, similar to that of 

 Taxocrinus and many of its allies. It differs in this from Sagenocrimts and Lithocrinus, 

 two of its Silurian predecessors, but agrees with Temnocrinus. The stem is large, and has 

 been found' incomplete to the length of 12 inches. The enlarged proximal columnals are 

 extremely thin in proportion to their diameter, as may be seen in the figures of F. saffordi 

 (Pis. XXX, fig. 16; XXXI, figs. 4a, 9), and very strongly crenulated. The union between 

 the proximal columnal and infrabasals was stronger than that between infrabasals and 

 basals, or succeeding connections, as already explained. The excess of organic matter in 

 the latter would cause this connection to give way first in fossilization, and therefore in dis- 

 integrated calices we often find the infrabasals adhering to the top of the stem. When 

 the stem breaks off it is usually at one or more joints distant from the calyx, the top columnal 

 remaining attached ; so that we scarcely ever in this genus see the inferior surface of the 

 infrabasals. This is well shown in Plate XXX, figure 50, where the proximal ossicle, is 

 attached to the base, but is so thin that the infrabasal sutures show through it. 



I camiot see in the facts now known sufficient evidence to sustain the theory that the 

 top columnal is a persistent proximale, and that new columnals were formed below it and 

 not next to the calyx as in the existing crinoids. The union of this upper columnal with the 

 infrabasals, while of the same type as that between columnals generally, may have been a 

 little stronger on account of its extreme thinness, and probably became practically a fusion 

 in many mature specimens where the interpolation of new ossicles had ceased. For discus- 

 sion of this point see the chapter on Morphology. 



Forbesiocrinus presents a singular case of divergence from the general rule regarding 

 the position of the small infrabasal. In a large number of detached bases of F. saffordi 

 showing the inner surface of the infrabasals, the small plate with but few exceptions is 

 located at the anterior instead of the right posterior as is usual in the Flexibilia. This is 

 also the case in several specimens of F. midtibrachiatus, but in other species in which these 



