NATURAL SCIENCES OP PHILADELPHIA. 139 



The genus Forbesiocrinus, proposed by Koninck and Le Hon, in 1854, (Re- 

 cherch. sur les Crinoides, p. 118,) was founded upon their F. nobilis, which they 

 think probably the same as Poteriocrinus? iiobilis, Phillips, originally included 

 by the latter author in his lsocrinitis, for which the name Taxocrinus was af- 

 terwards substituted, when he became aware of the fact that Isocrinus had 

 been previously used by Meyer for another group. In their description of 

 Forbesiocrinus, Koninck and Lehon characterize it as having five basal pieces, 

 alternating with five series of primary radials consisting of four pieces each, 

 with the anal and interradial spaces each occupied by from 12 to 13 pieces, 

 and the axillary spaces by three small pieces each. 



From all the descriptions and illustrations yet published, of the groups 

 Taxocrinus and Forbesiocrinus, it is therefore clearly evident that these two 

 types as understood by European authors, are distinguished by Taxocrinus 

 having but one or two ranges of interradial pieces, or none, and Forbesiocrinus 

 having from 12 to 13 of these pieces occupying each interradial spaces, and 

 a few small pieces in the axillary spaces above. In all other points of struc- 

 ture, and arrangement of parts, whether of the column, basal, radial or arm 

 pieces, they are understood and acknowledged to agree exactly. But as it 

 has been found that typical species of Forbesiocrinus, possess three more or 

 less developed basal pieces within or beneath those regarded as such by Kon- 

 inck and Le Hon, (Iowa Report, p. 628,) it might be supposed this character 

 would aid in distinguishing the two groups. It is well known, however, that 

 American typical species of Taxocrinus, without interradial or anal pieces, 

 or with but a single range of the two, such as T. Thiemei and T. juvenis,= 

 [Forbesiocrinus Thiemei and F. fuvenis, Hall, Jour. Bost. Soc. N. H. vii., 317 and 

 318,) possess precisely the same structure, being both described as having 

 small basal and subradial pieces.* Nor can we make the presence of inter- 

 axillary pieces, or the small patelliform supplementary pieces, so often seen 

 at the sutures of the radials in well defined Forbesiocrinus, a means of distinc- 

 tion, since neither are always present in otherwise typical forms of that 

 group with the interradial spaces filled with plates ; while well marked spe- 

 cies of Taxocrinus, such as T. Thiemei and T. juvenis, Hall, (sp.,) the first with- 

 out anal or interradial pieces, and the latter with the " interradial and anal 

 series consisting of one plate each," are described, the first as having " arm 

 joints showing the small patelloid plates very distinct," and the latter, with 

 " the small patelloid plates indicated by the strong curvature of the suture 

 lines of the radial plates, becoming more distinct in the arm plates." So if 

 we attempt to distinguish ihese groups at all, we must fall back upon the dif- 

 ference of the one group being without anals or interradials, or with but one 

 or two ranges of these pieces, and the other with a greater number. 



Now, however well this difference may serve to separate into two groups 

 the few known European species, an attentive study of our more nume- 

 rous American forms has shaken our confidence in the possibility of separa- 

 ting all the species hitherto discovered, into two sharply defined genera, 

 until some more reliable characters can be pointed out. Our reasons for this 

 conclusion will be better understood by consulting the following list of 22 

 American and European species and varieties, with a tabular statement of 

 the number of the various parts of each.f 



* Our Taxocrinus gracilis, of this paper, with a single series of interradials, shows clearly 

 three small pieces uuder the five considered the basale, by most authors. 



t We have not included Taxocrinus inter scapular is, Hall, (Iowa Report, pi. i. fig. 3,) in this 

 list, because it deviates from the typical forma of the genus, and seems allied to Dimerocrinus. 

 Phillips, though differing trom that genus in not having the arms composed of a double series of 

 minute interlocking pieces, as well as in some other points of structure. 



18H5.] 



