SAGENOCRINIDAE 23/ 



the posterior basal as usually seen in the fossil from the exterior. As the tube, in common 

 with all anal structures, for some unknown reason tends toward the right, it usually leans 

 in that direction, leaving the visible shoulder of the basal to the left the largest. The struc- 

 ture of the tube itself will be more appropriately given under Taxocrinus and Onychocrinus; 

 it is sufficient here to say that it passes up to the tegmen, and is bordered by perisome 

 extending on either side to the brachial series to which, as well as to the posterior basal, this 

 connecting perisome is attached, not by angular or straight faces but by smooth and rounded 

 margins. The general relation of the anal tube to the tegmen and the posterior brachial series 

 in Taxocrinus is perfectly shown in T. intermedins (PI. LIII, figs, la, b) , and in Onycho- 

 crinus by the various figures on Plate LXVII. 



The general form and character of the posterior basal as above described prevails gener- 

 ally in the family Taxocrinidae, and it is the morphological feature determining the structure 

 on which the family is to a large extent based. It may not always be so regular as in 

 Taxocrinus, and in some forms the tube socket is relatively higher, and may even be at the 

 upper margin ; this is the case, for example, in Synerocrinus, where the lower plate of the 

 tube is joined to the truncate end of the basal, and for part of its length to the brachials, on 

 account of which the genus is separated from the family. The two types of structure of the 

 posterior basal are thus absolutely and fundamentally different, and cannot be mistaken for 

 one another, although the condition of preservation of the fossils sometimes renders it diffi- 

 cult to interpret them with certainty. The only question need be, is the posterior basal 

 suturally connected with the succeeding plates, no matter how narrow the posterior area, or 

 how few or many plates are in it? If not so connected, it is of the Taxocrinus type. This 

 correlates with the test heretofore given, viz., the presence or absence of sutural connection 

 between the anal plate and adjacent brachials. 



As a necessary result of these structures the fact follows, often very important in prac- 

 tice, that the margins of the posterior rays bordering the anal interradius show marked and 

 constant differences in the two genera. In Forbesiocrinus these lateral faces of the brachials 

 are flat surfaces, marked to their full depth with either fossae and crenulated margins and 

 facets for connective tissue, or with interlocking grooves and ridges in a dorso-ventral 

 direction for similar connection (PI. XXI, figs. 1, 2, 3). They are angular all the way up in 

 the wortheni type, and part way up in the nobilis type, interlocking by sutural attachment 

 with solid interbrachial plates as thick as the margin of the brachial series. In other words, 

 the structure at the margin of the posterior interradius is the same as in all the regular 

 interradii. In Taxocrinus the structure of these margins is entirely different from that in the 

 other areas ; the lateral faces of the brachials are rounded and perfectly smooth, without any 

 kind of fossae, crenulation or rugosity for the attachment of plates, and the border of the 

 rays forms a straight line (Pis. XXI, fig. 4; LVII, figs. 4, 8a, b). They are adapted to the 

 attachment of finely plated perisome at their ventral margins, for which there is a slight 

 linear depression, but not of solid plates (PI. LVII, fig. 6b). Upon this character, for 

 instance, the type of Phillips's Taxocrinus nobilis (PI. LIV, fig. la and especially fig. lb) 

 can be readily distinguished from Forbesiocrinus nobilis with which it has been so much 

 confounded in the literature. If in any specimen the lateral margin of the rays bordering 

 the anal interradius, or of only one of them from the posterior basal up, is smooth and 

 rounded, without any indication of sutural attachment of plates, it does not belong to 

 Forbesiocrinus or any genus of the Sagenocrinidae. 



2. Correlated with these differences between the two genera in the structure of the 

 posterior basal, there is also a difference in the disposition of the interbrachial plates. In 

 Forbesiocrinus of the wortheni type these tend to fill up the spaces between the rays and their 

 divisions, running nearly to an apex, gradually diminishing in size and passing without any 

 abrupt line of demarcation into the tegmen (see various figures on Pis. XXVII-XXIX). 

 They thus form lanceolate areas more or less filled with plates suturally connected with the 



