380 SMITHSONIAN INSTITUTION 



several well-marked and abundant species. The form thus persisting is that of the Devonian 

 genotype, T. macrodactylus, having " four costals," or in our terminology a radial and three 

 primibrachs ; and it therefore necessarily holds the name Taxocrinus. The other form, 

 retaining the Silurian characteristic of a radial and two primibrachs only to the earlier part 

 of the Lower Carboniferous, has been removed to form the genus Eutaxocrinus. The modi- 

 fication from two to three primibrachs thus took place in the Devonian ; and among the 

 species of that age there is observed a tendency to variation, and some lack of constancy in 

 this^ character. By the time of the Burlington limestone the three-primibrach form, or 

 Taxocrinus, had become thoroughly established, and continued on through the St. Louis and 

 Kaskaskia to the extinction of the group ; whereas that with the two - primibrachs, or 

 Eutaxocrinus, had completely disappeared — the latest known species occurring in the lowest 

 member of the Lower Carboniferous, the Kinderhook. 



The species of Taxocrinus arrange themselves progressively into three not very sharply 

 defined groups, which nevertheless in a general way coincide with the development of the 

 especially strong characters of the genus, and its subsequent decadence ; these are based upon 

 the condition of the interbrachial system, as follows: (1) iBr wanting, Devonian; (2) iBr 

 few, with areas narrow, chiefly filled with perisome, Devonian to early part of Lower Car- 

 boniferous ; (3) iBr well developed, with wide areas and numerous strong plates, Lower 

 Carboniferous. 



The criteria for the closer discrimination of species are not in all cases as definite as 

 could be wished. This is especially the case with the Devonian species, in regard to which 

 we are embarrassed by scarcity of material, specimens for the most part being extremely 

 rare and in the English species very poorly, preserved. We have to rely to a large extent 

 upon characters which cannot be very accurately defined and do not lend themselves advan- 

 tageously to analytical statement ; such for instance as comparative size, the tapering of the 

 arms, etc. In the Lower Carboniferous, from which we have in most cases an abundance of 

 well-preserved specimens, the species are usually well-marked, especially toward the close 

 of that epoch. Nothing could be sharper and more satisfactory than the characters of such 

 notable species as T. colletti and T. ungula, or the distinction between the last three or four 

 small species with which the genus ended. The number of secundibrachs is not a reliable 

 character throughout, being subject to considerable variation in the Devonian species. In a 

 general way it may be said that the elongate species have 4 or more, and the short-crowned 

 species less than 4. In the later Lower Carboniferous the number of these plates becomes 

 more. constant, with a tendency to shortening of the intervals in the ray divisions, until in 

 the last surviving species the number is reduced to 2 IIBr almost without exception, and to 

 2 or 3 IIIBr — a condition closely paralleled in the latest species of Onychocrinus of the 

 same formation. 



Five species are recognized in the Devonian, and sixteen in the Lower Carboniferous. 

 The genus began feebly in the middle part of the Devonian, struggling along with a few 

 species while the antecedent Eutaxocrinus was running its course to extinction in the begin- 

 ning of the Carboniferous ; it then reached its acme in America during the first half of the 

 Lower Carboniferous with 3 species in the Kinderhook-Waverly ; 4 in the Burlington 

 (including Knobstone) ; and 3 in the Keokuk; it then entered upon its decline, finishing with 

 one small species each in the Warsaw, St. Louis, and two in the Kaskaskia. 



The relation of the species may be shown by the following analysis, beginning with the 

 earliest, and proceeding with a more or less interrupted progression to the end at the close 

 of the Lower Carboniferous : 



