TAXOCRINIDAE 375 



the left against the radial of that side and then to the right, the upper portion lying close 

 behind the edge of the posterior basal. Pressure of the tube against this radial plate, coupled 

 perhaps with the friction due to the opening and closing of the rays and the up and down 

 movement of the tegmen, sometimes produced a broad vertical groove on the margin of the 

 left posterior radial, which is a further striking evidence of the great mobility of the 

 Onychocrinus calyx (text-fig. 8). In most of the species the socket for the tube lies well to 

 the right side of the basal, and in some, such as T. lobatus, T. intermedins, T. ungula and 

 T. giddingei, the first plate is larger than those following it, and occupies an angular space 

 between the upper shoulders of the posterior basal and the right posterior radial ; in these 

 cases the plate is believed to represent the radianal, moved upward concurrently with the 

 elongation of the posterior basal to an upper oblique position. 



Our knowledge of the tegminal part of this structure in Taxocrinus is mainly derived 

 from the famous specimen of T. intermedins discovered by Wachsmuth and Springer in 

 1888 (Proc. Acad. Nat. Sci. Phila., p. 344), which first gave a clue to the true ventral struc- 

 ture of the Flexibilia. But the posterior aspect of the tube and its connecting perisome is 

 so perfectly shown in the specimens of Taxocrinus lobatus, T. ramulosus and T. whitheldi, 

 that we may consider it to be substantially the same as in Onychocrinus, where we have it 

 somewhat better preserved. The actual opening has not been seen in any specimen, and we 

 should scarcely expect to find it, being an aperture through the delicate perisome which is 

 always more or less folded upon itself and flattened. Otherwise the whole structure is com- 

 pletely and beautifully shown by my specimens of Taxocrinus and Onychocrinus, especially 

 a series of 0. ulrichi specially prepared for the purpose (PI. LXVII, figs. 2-6). The form 

 of the posterior oral is different in the two genera, and apparently the tube was longest in 

 Onychocrinus. 



The structure of the entire tegmen in Taxocrinus is perfectly shown in the specimen 

 of T. intermedins above mentioned (PI. LIII, figs, la, b). The orals are well rounded and 

 separated, and do not form such a distinct pyramid as part of them do in Onychocrinus. The 

 ambulacra are small and delicate, with covering plates transversely elongate — that is, short 

 and wide. This structure has been confirmed in a specimen of T. shumardianus (PI. LIX, 

 fig. 14&). 



Along with the above-mentioned characteristic anal structure go two other characters 

 also fully discussed under Forbesiocrinus, which must be remembered in trying to determine 

 to which of the two genera a specimen should be referred, viz. : (1) that in Taxocrinus one 

 or both margins of the brachials next to the anal interradius are smooth, rounded, without 

 markings or any specialized area for sutural attachment; and (2) that the interbrachials are 

 rarely suturally united to the rays higher than the last secundibrach, and that in the later 

 developed Taxocrini they slope away from the rays so as to form a concave distal margin, 

 instead of a lanceolate area wedged in between the rays as in the species of Forbesiocrinus 

 of corresponding horizon. In practice these facts are important, and often serve to settle 

 the identification in cases where the anal structure is invisible, or deceptively distorted by 

 pressure. The last feature is not so well marked in the Devonian and earlier Carboniferous 

 species, with their narrow interbrachial areas, while the contemporary Forbesiocrinus of 

 the nobilis type has the crescentic margin of the interbrachials though somewhat different 

 in detail. But in the species of the Upper Burlington, Keokuk, St. Louis and Kaskaskia, 

 where there is greatest occasion for comparison with Forbesiocrinus, the crescentic inter- 

 brachial margin is very conspicuous, while in the species of Forbesiocrinus from some of the 

 same formations the areas are invariably filled to an apex between the rays. 



In the matter of interbrachials the genus underwent a considerable progressive modifi- 

 cation. Certain Devonian species, among the earliest occurrences of the genus, have no 

 regular interbrachials, and apparently no place for any. Others, also in the Devonian but 



