Vol. 54.] 



MR. F. A. BATHEE OX PETALOCRINT78. 



435 



follows the law for the genus. We may describe the complete fan 

 as consisting of a normal fan divided into two almost symmetrical 

 halves M and E. by the ridge d, and of another normal half-fan L 

 of the same size and character budded oflf from M and separated 

 from it by the ridge s. M and E, differ only in the additional 

 forking of one branch in B., as denoted above by the asterisks. Or 



Pig. 14. — Diagram of the same. 



Fig. 13. — Petalo- 

 crinus expansus. 

 Tracing of arm- 

 fan. 



d 



[Nat. size.] 



we may regard L and M as forming two halves of a normal fan, 

 absolutely symmetrical about the i^dge s, so far as number of 

 branchings is concerned ; in this case R is the added half-fan 

 arising immediately from IBr. The one thing that we cannot do is 

 to regard the portion M as the intercalated one, as we found best in 

 P. longus ; the abnormality or the trend of evolution, whichever it 

 be, is different in kind in the two cases. 



The ventral surface of the arm-fan is almost quite flat, sloping 

 downwards only at the proximal end and the distal margin. Eidges 

 s and d are very broad and flat, of irregular width, and deeply ex- 

 cavate at the distal end. Eidge s shows clear traces of a furrow, 

 the original line of fusion, in both its proximal and its distal region ; 

 and it is this which favours the view that L is the added half-fan. 

 The broad and flat appearance of the other ridges is in part due to 

 weathering, but in any case their width increases towards the distal 

 end. There are no traces of covering-plates. 



Eolations to other Species. — That there is no real re- 

 semblance to P. longus has been shown already. The only question 

 is whether we are not dealing with an abnormal variation of P. vis- 

 bycensis ; and were the difference confined merely to the addition 

 of a half-arm, or even were this accompanied by some thinning of 

 the ridges to compensate for the increased width, one might answer 

 that question in the affirmative. But the variations with which 

 the abnormality is correlated are not such as appear natural con- 

 sequences of it ; they are greater width and flatness of the grooves, 



