238 SMITHSONIAN INSTITUTION 



brachials. In Taxocrinus this sutural connection extends for a relatively much shorter dis- 

 tance (rarely above IIBr 2 ) upward; and the interbrachials, instead of running to an apex 

 tend to spread out, ending in a crescentic distal margin of fairly good-sized plates, rising at 

 either end to meet the rays (PI. LIII, figs. 2-5, and various figures on Pis. LVIII, LIX). 

 This distal margin is smooth and rounded for the attachment of perisome (PI. LVII, 

 figs. 8a, b), leaving no place for further suturally connected plates; and in this respect the 

 difference applies to the F. nobilis type as well. This distinction becomes important in cases 

 like Taxocrinus giddingei and T. ungula, in which the perisome bordering the anal tube has 

 developed into rather large plates, so as to strongly resemble the posterior structure of 

 Forbesiocrinus; the form of the interbrachial areas determines their generic position. 



Forbesiocrinus is characterized by a great profusion of interbrachial plates ; while in 

 Taxocrinus, although not always absent as supposed by De Koninck and Le Hon in pro- 

 posing Forbesiocrinus, they vary greatly among species from few, or perhaps none, to such 

 abundance as is seen in T. whMeldi, T. colletti, T. ungula, T. giddingei, etc., which on 

 account of the profuse development of these plates have a considerable superficial resem- 

 blance to the later Forbesiocrinus. In both genera the matter of age must always be con- 

 sidered — the interbrachials being supplementary plates developed to fill up spaces in the 

 growing crinoid, so that a young individual of these same species may have scarcely any of 

 them (Pis. XXVIII, LV, LVI, LVIII). 



Transition forms showing intermediate stages of the two structures are to be expected. 

 The two species last mentioned are good examples of them from the Taxocrinus side, 

 especially T. ungula with its rounded anal series imbedded in good-sized plates extending to 

 the rays on either side (PL LV, figs. 3, 4, 6, 7) ; and the bordering plates are even more 

 developed in the diminutive T. giddingei (PL LIX, figs. 2, 3). In these species, and others 

 exhibiting to a lesser degree a tendency toward the Forbesiocrinus anal structure, the growth 

 of definite plates along the tube always begins at the right side, never at the left ; and in 

 specimens where this growth extends the farthest it is always considerably short of com- 

 pletion on the left side, the left posterior ray being connected by plates for only a short 

 distance. In none of such cases have I seen a complete development of these plates to the 

 top of the area on both sides, as in the Forbesiocrinus wortheni type. In these forms also 

 the tube-facet lies nearer the upper margin of the basal than in those where I have described 

 it, and its tendency is always toward the right upper corner. Doubtless if we could see the 

 exact form of the plates in some of these examples we should find a tendency to sutural con- 

 nection at the distal face of the basal, as we actually do in Synerocrinus. But a careful 

 inspection of the figures, and better still of the specimens themselves, gives a clear impres- 

 sion that these are only overgrown perisome plates, lacking the definite arrangement and 

 solid appearance of those in the other areas. 



On the other hand, in some otherwise typical Forbesiocrinus, e. g., F. multibrachiatus, 

 there is a tendency toward an arrangement of plates in a vertical series which, though 

 bordered on either side by perfectly solid plates, often develop a line of weakness by which 

 the series sometimes becomes partially detached along its left margin (Pis. XXVI, fig. 5; 

 XXVII, fig. 2; XXVIII, figs. 4, 5). These cases have hitherto added to the confusion 

 between the two genera, and some of the species have been shifted from one to the other 

 repeatedly. With our present understanding there is not much practical difficulty in assign- 

 ing them satisfactorily, the form of the interbrachial areas as above described assisting 

 materially, especially in questions arising among the later Lower Carboniferous species, 

 where the chief conflict may arise. 



In addition to the foregoing points of difference there is another of some importance 

 in the size of the visceral cavity, as shown by the relative massiveness of the calyx wall. 

 There is in both genera a gradual increase in thickness from the radials to the axillary primi- 



