120 Proceedings of the Royal Physical Society. 
and most imteresting series as regards the configuration of 
their dorsal fins; in the first both dorsals are short based, in 
the second the posterior dorsal has a base of considerable 
length, in the third the anterior dorsal is long based, while 
the base of the posterior one is still longer than in the second. 
The question here arises, What is the condition of .the 
dorsal fin-arrangement in the typical Phaneroplewron Ander- 
sont, Huxley ? for on that depends the validity of the new 
genus which I propose, provisionally at least, to institute for 
the reception of Whiteaves’s Ph. curtum. In the Dura Den 
fish the dorsal fin was originally described by Huxley, and 
afterwards by myself, as continuous anteriorly, and if that 
description holds good Ph. Andersoni occupies the fourth 
place! in the series of dorsal fin differentiations, and the 
Canadian species is also generically distinct. The specimens 
of Ph. Andersont in Edinburgh, or even in the splendid 
collection of Dura Den fishes at St Andrews, are not 
sufficiently perfect in the region to afford a decided answer, 
and I regret that since this question came under my notice, 
I have not had the opportunity of re-examining those in the 
London .collections, though here it must be noted that Mr 
Smith Woodward, in his “ Catalogue of the Fossil Fishes in 
the British Museum,” pt. ii., p. 247, states that the dorsal fin 
of Phaneropleuron is “single,” and it certainly appears to be 
so represented in Huxley’s figure of the finest specimen of 
Ph. Andersoni in that Museum. I feel therefore justified 
in provisionally erecting the Canadian species ewrtum of 
Whiteaves into a new genus characterised by its double 
dorsal fin, and for which the name Scawmenacia may be 
proposed. 
Mr Smith Woodward has recently pointed out? that in 
curt the proximal interspinous elements (axonosts) of the - 
anal fin are as in the second dorsal of Husthenopteron and 
Tristichopterus fused into one piece, which is followed by 
three distal elements (baseosts). This is well shown in the 
present series of specimens, but in one splendid example, 
! As a matter of evolution the series would of course be reversed, Phanero- 
pleuron being first and Dipterus Valenciennesit last. 
2 Ann, and Mag. Nat. Hist. (6), vol. xi., p. 241. 
