482 



ME. F. A. BATHER ON PETALOCRINFS. [Aug. 1 898, 



fan as composed of three portions, L, M, and E., separated by the 

 two ridges s and cZ, and each beginning with IIBr. 



In portion E, the branchings follow the law for the genus. There 

 are ten finials, of which the two internal and the two external 

 (using those terms througliout with reference to the whole fan) are 

 YIBr, the others being YBr. The finials, especially the YBr, are 

 very long, some occupying fully three-quarters the length of the 

 fan. The ridges are narrowest about halfway down the arm, and 

 thence gradually widen distalwards. Thus a single ridge narrows 

 from '4 to '2 mm., and then widens to 'O mm. The grooves, 

 which are usually about 'o mm. wide, may be compressed to 

 •25 mm., and widen distally to "65 mm. The ridges are always 

 narrower than the immediately adjacent regions of the grooves that 

 they separate. 



Pig. II. — Petalocri- 

 nus longus. Outline 

 and branchinc/ of 

 arm-fan, x 2 diam. 



Pig. 12. — Hypothetical diagram of the same. 



s & d, sinistral and dextral 

 ridges ; L, M, & R, left, 

 median, and right por- 

 tions ; I & r, left and 

 right subdivisions of M. 



In portion L the branchings follow the law for the genus, except 

 that two rami of one dichotom (the internal IVBr, i. e. next 

 portion M) both branch at the same level. The preserved finials 

 are all YBr ; but since a large part is missing at the distal end, we 

 are free to suggest that the internal branch probably, and the 

 external almost certainly, forked again. In that case the finials of 

 portion L would precisely correspond to those of portion R. 



Portion M is abnormal in its branching : first, in that it does not 

 dichotomize into divisions that are at all equal, but forks into a left 

 division with five finials (I), and a right division with two finials (r) ; 

 secondly, in that the levels at which branching takes place cannot 



