404 SMITHSONIAN INSTITUTION 



Taxocrinus praestans n. sp. 



Plate LV 1 1 1, figs. 1-8 

 The largest Taxocrinus known, five adult specimens averaging 70 mm. 

 high by 45 wide; base 13 mm. It is one-third to two-fifths larger than the 

 largest specimen of T. colletti, which it resembles in general form and propor- 

 tons, but from which it differs decisively in the stem; this, instead of dimin- 

 ishing in size all at once by a sudden constriction where the thin column.als 

 end, tapers gradually to the minimum for a considerably greater distance, 

 passing imperceptibly into longer and alternating ossicles. There is also no 

 indication of any peripheral denticulate ornamentation, and the stem is pre- 

 served in three specimens for a sufficient distance to show that it has the ordi- 

 nary form for the genus, like that of T. ungula. This distinction is confirmed 

 by several minor differences. The brachials throughout are relatively nar- 

 rower, and especially deeper ; the interbrachial areas wider and the plates more 

 numerous. The IIBr are almost uniformly 4 instead of 3; the five specimens 

 are all free from the matrix, exposing 50 rami, of which 43 have 4 IIBr and 

 7 have 3 ; so that instead of a proportion of 58 per cent with 3 IIBr, we have 

 here 86 per cent with the longer division. The basal plates have a peculiar 

 shape; the apex is prolonged up between the radials like a narrow tongue, and 

 the basi-radial sutures are sinuous, so that the proximal face of the radials 

 is broadly curved instead of angular as usual; this may be seen in figures 1, 

 2, 3 of Plate LVIII, and is the case in another large specimen not figured; it is 

 not so in figure 4, of a specimen from a different locality. 



In none of the specimens is the anal side preserved intact, all being injured or distorted 

 in that part ; from what is left it is evident that the tube itself was not very strong, but was 

 bordered by unusually strong perisome at the right side as in T. ungula, while at the left 

 the perisome must have been weak, as no remnant of it is found next to the left posterior ray 

 in any of the specimens. The interbrachial development is in accordance with the mature 

 stage represented by the large specimens, and the broadly curved crescentic distal margin 

 with perisome following is very conspicuous. Four of the specimens are from a single 

 colony in the Indian Creek beds ; they are remarkably uniform in size, the fourth being as 

 large as the three which are figured, 1, 2, 3 ; the other one, figure 4, is from the closely equiv- 

 alent beds in Washington County, Indiana. I have figured on the same plate with these 

 several young specimens found in the same locality, some of them from the same colony ; 

 they may or may not belong to this species, not having the stem preserved to determine it 

 except in figure 8, which has rather that of T. colletti but without any peripheral ornament. 

 No intermediate stages of this species were found, and one need only compare figure 1 

 on Plate LVII, of a specimen from the same locality, with the four large figures on Plate 

 LVIII, to see that there is no reason for thinking that the much smaller T. colletti is the 

 younger stage. 



Types. Author's collection. 



Horizon and locality. Lower Carboniferous, Keokuk Group, below the Crawfordsville 

 beds ; Indian Creek, Montgomery County, and Canton, Washington County, Indiana. 



