III. Thenarocn'nus calJipygus. 233 



a Family CARABOCRlN'lDiE, characterized by the presence in 

 the basal circlet of a radiaiial or part of one, is a different 

 matter. The Fistulata are now so well known that their 

 classification must depend on the question of descent. In the 

 present instance, however, this question is obscured, partly 

 because so few species of the genera in question are known, 

 but chiefly because those genera are early forms but a little way 

 removed from the common ])arent stock. Carahocrinus, for 

 instance, seems related to Eiisjn'rocriyms, of which genus a 

 species, E. ohconicus^ has been found by Mr. W. R. Billings* 

 in the Trenton Limestone. Thenarocrinus s\s,o presents some 

 points of resemblance to EuspirocrinuSy especially in the arms, 

 in the general shape of the dorsal cup, and in the column. 

 But both Carahocrinus and Euspirocrinus are very closely 

 connected with Ottaicacrinus and with early species of Den- 

 drocrinus. In fact, were we to consider Ordovician forms 

 alone, we should undoubtedly place all these genera in one 

 Family. Clearly, however, this would not be satisfactory; 

 the evolution of that assemblage did not cease, and the ques- 

 tion is — Can we discern more than one line of evolution ? 

 Certainly there seem to be three divergent lines ; and the 

 fact that two of these (the Carabocrinida^and Euspirocrinidi\3) 

 soon appear to reach their termini does not impugn their 

 existence. 



Undoubtedly the establishment of a Family Carabocrlnidoe 

 would appear more reasonable if we could trace its descent 

 rather further than is at present possible, but among forms 

 reckoned as Fistulata the descendants of Thenar ocrinus are 

 still to seek. There is, however, a likeness so remarkable 

 that it cannot be overlooked. The resemblance of Thenaro- 

 crhius to Enallocrinus may be suj)erficial, but, except for the 

 anal structures, it is very complete. Tlie plates of the dorsal 

 cup, other than anals, are tiie same in number and in shape, 

 and the following sentences from the most recent description 

 of Enallocrinus^ ^PP^J almost equally well to Thenaro- 

 crinus : — " First radials wide, their distal faces usually occu- 

 pied by a deep lunate excavation in which the second primary 

 and one or two higher radials rest ; sometimes, however, 

 truncate." " Rays completely disconnected from the first 

 radials up, and the arms becoming free variously between the 

 first to the fourth bifurcation. Second radials \i, e. first 

 costals] perforated by a large axial canal which passes down- 

 ward ; it ramifies within the higher radials, and passes into 



* Trans. Ottawa Field Naturalists' Club, ii. no. 2, 1S85. 

 t Wachsmuth and Springer, "C/-ote/ocr<'MM5 : its Structure and Zoolo- 

 gical Position," Proc. Acad. Nat. Sci. Philadelphia, 1888, pp. 387, 388. 



