ICHTHYOCRINIDAE 327 



This complete elimination of disturbing elements, combined with the distinct heterotomy 

 of the arms, forms a high degree of specialization, under which there is some variation in 

 characters elsewhere of generic importance. One of these is the presence or absence of inter- 

 brachials. Ordinarily the function of interbrachials is to fill spaces in the expanding calyx 

 of the growing crinoid ; therefore they increase in number with the maturity of the indi- 

 viduals, and may be expected to be fewest or entirely absent in the smaller specimens. But 

 that rule does not hold good in Wachsmuthicrinus. The Knobstone species, although attain- 

 ing a large size, are uniformly without any trace of interbrachials ; the Lower Burlington 

 species, as a rule, have them ; — not only the large W . bernhardinae and W . spinifer, but 

 also the smallest specimens of what have always been considered W. thiemei (PI. XLIII, 

 figs. 6, 7, and about 15 others not figured). But the type and another one very similar to it 

 (figs. 1, 2), two of the largest specimens of that species ever found, are without any definite 

 interbrachials, and some of the rays interlock perfectly for a considerable distance. These 

 specimens are all from the same locality, and have such an entirely similar facies in other 

 respects that they cannot be separated specifically on the difference of interbrachials. 



It is evidently here not a question of individual growth so much as of sporadic variation, 

 so that a character ordinarily of generic value is subordinated to specialization in another line, 

 and is worthless. The variations in W . thiemei have been the subject of frequent comment 

 in the various discussions of the genus Taxocrinus, and were also the cause of the reference 

 of some of the species to different genera. W. spinosulus was described by Miller and 

 Gurley as an Ichthyocrinus solely on account of its having no interbrachial plates. There is 

 also within the genus, as in Dactylocrinus, a tendency to modification of the arm branching 

 in the direction of dichotomy, by an increase in the relative size of the ramules, and of the 

 intervals between them as in the Knobstone species (fig. 15). Another illustration of the 

 tendency to variation in this genus is found in the growth of nodes and spines on the axil- 

 laries, especially the axillary primibrachs where they sometimes become of enormous size. 

 This also is not a matter of individual growth, but it is either a specific character or a purely 

 sporadic variation, as some of the smaller specimens have ample spines, and some of the 

 largest only very small nodes, or none at all. 



The basal structure brings the genus clearly within the family Ichthyocrinidae. The 

 infrabasals lie within the basal ring, completely under the column (PI. XLIII, figs. 3, 5, 7). 

 They become in some species relatively somewhat larger than in any other genus of the 

 family, and occasionally tend to develop a slight flange-like extension over a small part of the 

 basals. but none extending to the limits of the column facet. 



Wachsmuthicrinus is restricted to the earliest formations of the Lower Carboniferous, 

 occurring first in the Knobstone of Indiana, Kentucky and Missouri, and ranging throughout 

 the two beds of the Burlington limestone. W. thiemei, of the Lower Burlington, is well 

 represented in the collections made at Burlington, though never abundant, and it has been 

 identified from New Mexico ; all the others are extremely rare. The genus probably occurs 

 in the British Lower Carboniferous, from which I have figured a defective specimen. The 

 tendency to variation already mentioned makes the differentiation of species rather unsatis- 

 factory ; either several new ones must be recognized among specimens from the same locality 

 and horizon, or a wide latitude must be admitted for what seems to be the leading species in 

 each formation. The latter course seems preferable. This leaves one species from the Knob- 

 stone, four from the Burlington, and one, not very characteristic, from Scotland ; these are 

 arranged as follows : 



