374 SMITHSONIAN INSTITUTION 



Taxocrinidae with rays not abutting over interbrachial areas. Infrabasals 

 low, forming but a small part of the calyx wall; sometimes entirely concealed by 

 the column. Posterior basal elongate. Radianal, if present, only in upper 

 oblique position. Interbrachials variable, usually present in lower part of area; 

 frequently numerous, with distal margin crescentic, rising toward the rays; 

 sometimes wanting. Primibrachs three. Arms dichotomous, divergent. Col- 

 umn usually enlarging proximally. Tegmen composed of pliant skin with cal- 

 careous spicules imbedded, traversed by calcified ambulacra passing between 

 parted orals to an open mouth. Anal opening excentric, at the end of a tube 

 formed by extension of the ventral perisome and supported at the posterior side 

 by a vertical series of anal plates. 



Genotype. Cyathocrinus ? macrodactylus Phillips. 



Distribution. Middle Devonian to close of Lower Carboniferous; Great 

 Britain, Belgium, and the United States. 



Taxocrinus is a perfectly typical genus of its family. The structure of the posterior 

 side, with its anal tube originating on the posterior basal sometimes far down upon that 

 plate, and connected with the rays on one or both sides by perisome only, is in complete con- 

 trast with that of Forbesiocrinus, with which it has been most frequently confounded. The 

 details of this structure have been fully given in the discussion under that genus, and it is 

 unnecessary to repeat them here. Something further may be added, however, as to the 

 so-called anal tube. 



Although the vertical series of plates in the posterior interradius in this and allied 

 genera has been sometimes called an " anal tube," there has been a tendency arhong authors 

 to use the term with qualification and caution, so as to avoid the meaning of an actual tube, 

 it being generally assumed in later years that this structure does not represent such an organ. 

 My own investigations, with material and preparations not accessible heretofore, have shown 

 that the long familiar term expresses the facts better than any other, and may be properly 

 used in description without circumlocution. 



While the plates which constitute the vertical series as we see it in a dorsal view are not 

 themselves hollow, yet they form the support for a fold in the perisome which is in every 

 way analogous to the passage for the hind gut. The organ thus formed is a jointed struc- 

 ture, and therefore flexible notwithstanding the solidity of its plates ; composed on the dorsal 

 side of a series of heavy, rounded, elongate plates, which are articulated with one another. 

 In the best developed forms of Taxocrinus, as well as in the closely allied Onychocrvmts, 

 where the tube originates half way down on the outside of the posterior basal, the lower 

 plates to the extent of one or two are perfectly solid, and rest within a socket and groove in 

 the posterior basal. Above this the plates become ventrally crescentic, and the perisome is 

 attached to their lateral edges by a distinct line of depression cut out for its accommodation 

 (PL LXVII, figs. 4, 6). A portion of the pliant tegmen is gradually lifted up between the 

 posterior oral and the anal series until it protrudes in the form of an inverted funnel-shaped 

 passage ; it thus comes to form the ventral side of an elevated tube for the exit of the rectum 

 and the discharge of excrement away from the oral center. There are in some species from 

 7 to io of these heavy, jointed plates, and in Onychocrimts from io to 13 — the terminal one 

 being rounded, and appearing as if attached to the perisome (PI. LXVII, fig. .36). 



As found in the fossil state in the latter genus, with the tegmen invariably compressed, 

 distorted, or collapsed and sunk down into the dorsal cup, the tube is often curved first to 



