1000 The American Naturalist. [November, 
tinctly segmented, while in the Symmorium this element forms a sin- 
gle piece, except possibly at the extremity. According to Traquair 
there is an “ oblong” proximal segment of the metapterygium “ whose 
anterior portion seems to have absorbed the bases of one or two adja- 
cent radials.” In Symmorium reniforme, all the basals (radials of 
Traquair), are fused at their bases with the metapterygium. The 
basals are also more numerous than in Dr. Traquair’s shark, for he 
says “some small radials are seen attached to the preaxial side of the 
first two segments—none on the others.” My specimen agrees with 
Traquair’s in the absence of basals (radials) from the post-axial side of 
the metapterygium, where indeed they are not to be looked for. 
The structure of the paired fins here pointed out, sustains the views 
already announced by Dr. Bashford Dean’ in a recent paper, and this 
author is to be congratulated that the view which he has put forth, is 
so fully sustained by the material in my possession. One hypothesis 
which he holds requires further confirmation; viz, that the metaptery- 
gium is formed by the fusion of the basal elements. The extensive 
fusion seen in the later genus Symmorium as compared with the earlier 
genus Cladodus, supports his position so far as it goes, but the origin of 
the primitive metapterygium is not thus explained. 
My observations on Symmorium, together with those of Traquair, 
Jekel, and Dean, show that the median axis of the archipterygium is 
not propterygial or mesapterygial, but is metapterygial. This greatly 
simplifies the conception of the history of the Selachian fin, where the 
metapterygium supports the greater number of the other segments. 
It shows that the Ichthyotomi are not elements in the phylogeny of the 
sharks,’ but form a side branch. It is further to be observed that the 
essential distinction now discovered hetween the metapterygial and 
other elements of the paired fins, must be maintained in our future 
studies of them. A clear distinction between baseosts and axonosts in 
the paired fins has been hitherto wanting. For the present it may be 
convenient to regard the metapterygial elements as axonosts, and those 
which have originally been branches of that axis, as baseosts. The 
scapular base of the Selachian fin consists then of one axonost and two 
baseosts. The typical Actinopterygian fin will have as its scapular 
base, according to Gegenbaur’s homologies, baseosts only, the metap- 
terygial (axonost) elements having entirely disappeared. 
It results from the preceding observations that the Cladodontide 
must be removed from the Ichthyotomi where Dr. Woodward placed 
"Transac. N. York Academy of Sciences, 1893, April, p. 124. 
*See Proceeds. Am. Philos. Soc., 1892, p. 280. 
