MELOCRINIDJE. 309 



Austins,^ who recommended its application for certain species of Platijcrinus 

 with " central valvate unobstrusive mouths^ or mouths capable of being 

 withdrawn into the visceral cup." Even substituting " anal opening " for 

 mouth, there is no Platycrinoid to which the above definition is applicable ; 

 besides the name has never been accepted by any writer, nor was it applied 

 to any particular species by Austin himself. Centrocrinus, as here defined, 

 is allied to the Platycrinidae in its anchylosed large basal disk and the small 

 size of its costals, but differs essentially in having two rows of interbrachials 

 within the limits of the dorsal cup. Lyon's figures show interbasal sutures, 

 but they cannot be seen in his specimens. 



Centrocrinus pentaspinus (Lyon). 

 Plate XXV, Figs.^lOa, h, c. 



1869. Actinocrihus pentasphius — Lyon; Trans. Amer. Pliilos. Soc, Yol. XIII. , p. 453, Plate 26, Pigs, d 



and dL 

 1881. Centrocrinus pentaspinus — W. and Sp. ; Kevision Paleeocr., Part II., p. 104 (Proceed. Acad. Nat. 



Sci. Phila., p. 279). 

 Syn. Actinocrinus multicornus Lyon; Trans. Amer. Philos. Soc, Yol. XIII., p. 455, Plate 26, Pig. 



e; and Centrocrinus multicornus — W. and Sp. (Revision, Part II., p. 105). 



Dorsal cup as wide as high, subcylindrical, flat below, the sides at right 

 angles to the bottom. Plates without ornamentation, but the radials, and 

 sometimes the first interbrachials and first costals, armed with a sharp spine, 

 while the other plates are merely convex. 



Basal disk very large, forming a regular pentagon, whose surface is 

 covered by a sort of rounded collar, extending over the greater part of the 

 surface, leaving only the angles free. Eadials almost twice as large as the 

 two costals together, the upper face much narrower than the lower. First 

 costals quadrangular, half the width of the radials, and less than half their 

 length; the second costals shorter than the first, pentagonal, with obtuse 

 upper angle. Distichals two, short, curved like arm plates; the upper ones 

 excavated to form the arm openings, which are large and arranged in groups 

 with wide interspaces. Interbrachials four ; the first plate large, hexagonal, 

 rising to the top of the first costals, the lower angle deeply wedged in be- 

 tween the radials; the two of the second range are followed by a small piece, 

 resting between the arm bases. Ventral disk, arms and anus unknown. 

 Column small ; axial canal apparently circular. 



* Thomas Austin, and Thomas Austin, jun., 1843, Monogr. Rec. and Poss. Crin., p. 6. 



