BY F. RATTE, IN T G. DES ARTS ET MANUP., PARIS. 1075 



of the " Interradial, Interaxillary and Interbrachial Plates " 

 (p. 237), and in the chapter treating of the " Ventral Perisonie" 

 (p. 281). 



Although it might appear natural to extract and include here 

 the revised generic diagnosis of Tribrachiocrinus, I think that it 

 does not sufficiently differ from my description, while it takes 

 nearly the whole of page 175 of the 'Revision.' I think it more 

 useful to give here a glance at the classification in order to point 

 out the place of this genus in the Paleeocrinoidea. I may remark 

 here that I now use the revised orthography of the generic name 

 which needs no comment, the first spelling being obviously 

 erroneous. 



The generic name Tribrachiocrinus was proposed by Professor 

 M'Coy on the supposition that there were only three arms ; but, 

 if according to Wachsmuth and Springer, the two ankylosed 

 brachials supported two arms each more than mere rudimentary, 

 then our fossils would have had three large and four smaller arms. 

 This being so, the name etymologically considered, is now a 

 misnomer ; nevertheless it conveys to the mind the notion that 

 there were three conspicuous arms. 



In the ' Challenger Report,' (Zoology, Vol. IV. pp. 149-154,) will 

 be found, according to Dr. Carpenter, the distinctions between Neo- 

 crinoidea and Paiseocrinoidea. Among these much stress is laid, 

 according to the 'Revision' (1) upon the symmetry of thecalyxinthe 

 Paiseocrinoidea, which Carpenter attributes to the intercalation of 

 an anal plate. To this there are many exceptions. But, among 

 other differences there is one absolute, that in the Paiseocrinoidea 

 the mouth and disk ambulacra are completely closed, while in the 

 Neocrinoidea the ambulacra have open food grooves. 



(1) Rev. Part III., p. 294. 



