90 SMITHSONIAN INSTITUTION 



family between Cyathocrinus and Gissocrinus. The fact that both of the Tren- 

 ton genera in question are in the same primitive stage in the development of 

 the radianal, a character which is now shown to be of the very highest impor- 

 tance in the phylogeny of the crinoids, adds g'reatly to the significance of their 

 close approach in otber characters. It should be added that the two species of 

 Cupulocrinus are extremely abundant and range throughout the principal Cana- 

 dian localities and southward to Kentucky, while Protaxocrinus is one of the 

 rarest fossils in these rocks. 



Having, therefore, two contemporaneous genera in the earlier Ordovi- 

 cian, existing at ' the same locality and horizon, and being in the same 

 morphological condition in regard to the one strongest and several of the minor 

 characters; the one flourishing in profusion and the other extremely rare; we 

 have the very conditions under which we might expect to find evidence of evo- 

 lutionary changes marking the divergence of two higher groups which are 

 admittedly of a common origin. The divergence in these two forms is of so 

 simple a nature, and by such close steps, that we need not look so very far back 

 in the geological scale for a probable common ancestor; and we may yet hope 

 to find it in the Ordovician. 



A somewhat analogous convergence of some characters is to be noted be- 

 tween the forms just discussed and the Camerate family Reteocrinidae, as 

 exemplified by such species as Reteocrinus o'neali (PI. LXXV, figs. 7, 8). In 

 the pliant tegmen passing down into the interradial areas, and the strong ver- 

 tical series of anal plates leading to an opening through the perisome, there is 

 certainly a marked similarity. But except for its pliability there are in these 

 forms none of the characteristics of the Flexible tegmen, such as the exposed 

 mouth and ambulacra. I have a hundred or more specimens of R. nealli ex- 

 posing the tegmen in a similar condition to that shown in these figures, and I 

 have studied them with great care to find a trace, a survival, or a premonition 

 of mouth or ambulacra, but without success. And it seems to me that these 

 organs must be regarded as definitely in the subtegminal stage of the Camerata, 

 but with transitional phases toward or from the Flexibilia. This form belongs 

 to a later period — Silurian — but another species of Reteocrinus is of the same 

 age as the principal species of Protaxocrinus and Cupulocrinus above discussed ; 

 so that all three types may have coincided in time and place as representatives 

 of the transitional tendencies out of which the three greater divisions of the 

 crinoids were evolved. 



Another clearly transitional form, also from the same older Ordovician 

 formation, is the genus Cleiocrinus. Its position seems to lie between the 

 Flexibilia and the Camerata on the one hand, and between these and the 

 cystids on the other. In regard to the calyx, the general habitus of the speci- 

 mens, the articulate structure and flexible walls, point strongly toward the 

 Flexibilia of the type having a solid posterior interradius and no anal tube ; yet 



