iOO Dr. F. A. Bather on British Fossil Crinoids : 



sliown. The petal-floors are shaped as iu B. diductylus, are 

 flat, and not so much depressed. Tiie peutagoual lumen, 

 with radial angles, has a diameter of about '2 mm. 



In such a joint-face the essential feature that gives the 

 Bulanocrinus character is the distinction between the peri- 

 pheral crenellae and those of the ridge-groups as marked by 

 the sudden change of length and of angle. Thus the angle 

 made by the most adradial of the peripheral crenellae with 

 the one next it is about 32°, but that made with the adjacent 

 crenella of the ridge-group is about 55°. There is actually 

 more distinction in this way than was noted in B. didactijltts. 



The actual number of peripheral crenellae is less than in 

 B. diductylus^ and, since in each case we are dealing with 

 full-grown examples, this is not due to the size of the 

 columnal. The length of those crenellae relative to the 

 diameter of the columnal is less than in B. didactylus, and 

 the transition from the interradial length to the adradial is 

 more gradual ; the shortness is rather a Bulanocrinus cha- 

 racter, but the gradual transition is as in Isocrinus. 



As representing the Isocrinus type of normal joint-face 

 may be taken one of the well-preserved fragments E 426 a 

 (Fig. 6). This is subcircular, tending to pentagonal with 

 rounded angles. Mean diameter 4'1 mm. Peripheral 

 crenellae not more than 7, and less if the adradial ones be 

 reckoned with the ridge-groups, into which they merge 

 insensibly; length gradually passing from "5 mm. inter- 

 radially to '7 mm. adradially ; evenly spaced, confluent 

 externally, sometimes subconfluent internally. Ridge - 

 groups : outer adradial crenellae only 3 or 4 in all, gabled 

 alternating, with no sudden change of angle or size from 

 the peripheral crenellae ; at 1*0 mm. or less from the peri- 

 phery they change into the parallel radial ridges, which are 

 relatively wide ('8 mm.) and meet in a rather large central 

 area only slightly less raised than themselves. The radial 

 canal is rarely seen, for these specimens have no radial 

 depression or pore apparent. Lumen pentagonal, with a 

 diameter '3 mm. or less. Petal-floors kite-shaped, outwardly 

 depressed, but rising towards the central area ; they are 

 pitted by minute canals for the passage of the ligament 

 fibrils, dispersed over the middle of the floor, but on each 

 side forming a straight series, which continues on to the 

 central area, where the two series end in a pair of slightly 

 larger canals. In vertical section these canals can be traced 

 following a straight course through the stereom, which has 

 a marked vertically fascicular structure, slightly denser at 

 half the height of the ossicle where it was first formed. 



