NATURAL SCIENCES OP PHILADELPHIA. 141 



From this statement, it will be seen that, starting from such forms as Taxo- 

 crinus poly daclylus, T. Thiemei, &c., without interradial or anal pieces, we pass 

 by a very easy gradation through the young of T. communis, having a single 

 range of granules representing interradials, to the adult of the same, with 

 one well developed interradial in each space, and one anal surmounted by 

 two or three granules. Then we have several other species, with a single 

 range of interradial and anal pieces, after which we pass to T. ramulosus, with 

 one interradial, three to six anals, and three interaxillary pieces, and then to 

 T. tuberculatum, with two interradial and two anal pieces in each space ; after 

 which we have F. asterixformis, with its four interradials and four anal pieces. 

 Continuing down the list, we find, as we pass from species to species, the 

 number of these intermediate plates increasing as gradually as we could ex- 

 pect these pieces in species of the same genus of crinoids to do, until we ar- 

 rive at F. Worlkeni, with its thirty or more interradials, and twenty or more 

 anals. Nor is it probable this is the maximum number of these pieces some- 

 times visible between the rays, since it is known that in some species, such 

 as F. exculptus, Lyon and Casseday (sp.), they continue on up, and pass with- 

 out change or interruption, into a series forming a solid dome above. Hence 

 it is probable that in the typical forms of Taxocrinus, without interradials, or 

 but one or two rauges, and long ponderous rays, such as T. macrodactylus, T. 

 polydactylus and T. ramulosus, the visceral sac corresponded in size with 

 that of the column and rays, or in other words, extended as far up at least as 

 the second bifurcation, and that it was merely protected by a dermal enve- 

 lope between the rays aud above. The fact that this integument protecting 

 the softer parts, may have, in some instances, merely secreted a rudimentary 

 piece at the bottom of each interradial space, or one or two well developed 

 plates, or filled the whole space partly or entirely, or continued the process 

 of secreting calcareous matter, until the whole summit was arched over with 

 a solid vault, although probably presenting in the various degrees of this 

 process good specific differences, can scarcely, we should think, be regarded 

 in this group as presenting sufficiently important characters for the distinc- 

 tion of genera, especially when these differences are not coincident with any 

 other peculiarities. In addition to this, when we bear in mind that different 

 sized individuals of the same species, as in Forbesiocrinus Agassizi and F. ez- 

 culptus, the number of interradial pieces are acknowledged to vary in the first 

 instance from fifteen to twenty five, and in the second from twenty to twenty- 

 five, we can readily understand that some caution is necessary in basing even 

 specific distinctions on these differences alone. 



Nor on the other hand, if we direct our attention to the primary or second- 

 ary radial pieces, or to the arms, do we see anything to sustain the generally 

 accepted distinction of two genera in this group ; for, if we make the radial 

 series, for instance, a basis of classification, we would have, as may be seen 

 by glancing at the 5th and 6th columns of the foregoing table, to place Taxo- 

 crinus nuntius, without anals or interradials, not only along with Forbesiocri- 

 nus spimfer, with its single range of interradials, but with F. Agassizi, -with its 

 thirty to forty anals, and twenty-five to thirty interradials. The same me- 

 thod would also place a variety of F. Meeki with seven interradials in the 

 same group with Taxocrinus juvenis, Hall, with its single range of interradial 

 pieces. 



Now from these facts, it must be evident, we think, that if Forbesiocrinus is 

 to be retained as a distinct genus from Taxocrinus, it will have to be separated 

 upon some characters or differences not yet observed. Hence, although we 

 shall continue to use the two names, for the sake of convenience, ranging un- 

 der Taxocrinus species without interradial or anal pieces, as well as those 

 with one or two of each ; and under Forbesiocrinus, those with a greater num- 

 ber of these pieces, we shall do so at least until better evidences of their 

 being distinct genera have been adduced with the understanding that we 



1865.] 10 



