FINS OF ELASMOBRANCHS. 477 
Altgether, valuable as are such suggestions as hints for the direction of inquiry, 
I do not think we are yet in a position to do more than guess at the exact form and 
precise mode of origin of the archipterygium. 
There is also the objection that something may be said in favour of the line of the 
propterygium having furnished the air-breathing limb-axis. 
Before entering upon this point, however, it may be well to review Professor 
Huxley’s! special interpretations of fin-homologies. 
In the first place, he regards the Notidanus-pectoral as the least modified form of 
Elasmobranch pectoral, that in which the Ceratodus type is least departed from. The 
mesopterygium he regards as the remains of the shrunken axis; the propterygium he 
appears to deem a modified preaxial ray; and the metapterygium he considers? as 
“ formed by the coalescence of the axial ends of the pectoral rays.” 
He does not, however, explicitly refer to the small cartilage called by Gegenbaur 
propterygium, and figured by him®, and found by me (Plate LXXV. fig. 3, a) to be 
divided off from the bases of the preaxial rays by a preaxiad extension of the meso- 
pterygium. Moreover I did not find the most preaxial ray much “ broader than the 
others,” or “ two-jointed,” but distinctly divided into four segments, while Gegenbaur 
represents it in WN. griseus* as much narrower than the 
others, and with only one joint. In WN. cinereus® he repre- 
sents it with five segments and a very small, still more pre- 
axiad cartilage annexed to it very near its base. Evidently, 
then, there is much individual variation in these parts, as it 
might well be expected @ priori that there would be. I 
cannot, therefore, regard the above explanation of Notidanus 
by Ceratodus as more than an ingenious speculation. 
In Cestracion® we have, as we have seen, a great cartilage 
which evidently represents both the pro- and mesopterygium 
undifferentiated, or coalesced; and we have evidence of the 
great plasticity of those structures in the appearance of three 
large cartilages connected (two directly and the third by these 
two) with the large compound cartilage. 
That this great cartilage is so complex in nature is made 
evident, I think, by the pectoral of Scymnus lichia’, in which 
one great cartilaginous mass must answer to the pro-, meso-, 
and metapterygium, all three. 
Profesor Huxley next speaks of Scylliwm*, which has a greatly reduced mesoptery- 
gium (still considered, of course, by Professor Huxley as the reduced limb-axis) and a 

Pectoral fin of Seymnus 
lichia. 
. P.Z.8. 186. 2 LZ. ¢. p. 50. % Untersuch. ii. plate ix. figs. 1 & 2. 
4B. c. fig. 1. Pearce, Lie ° Huxley, /. c. fig. 11, page 51, and Gegenbaur, pl. ix. fig. 3. 
7 Gegenbaur, plate ix. fig. 9. ° ZL. ¢. fig. 10, p. 48. 
