1885.]} Progressive and Retrogressive. 237 
man,’ which is apparently nearly related to the Cladodonts of the 
Devonian seas. This genus has more numerous branchial slits than 
all but two of the genera of existing sharks, and it differs from all 
but these two in having a more perfect articulation between the 
tooth-bearing bones and the cranium. Of the Hyopomata a 
much clearer history is accessible. It has three primary divisions 
or tribes which differ solely in the structure of the supports of 
the fins. In the first division, the Crossopterygia, the anterior 
limbs have numerous actinosts which are supported on a pedun- 
cle of axial bones. The posterior limbs are similar. In the sec- 
ond division, or Chondrostei (the sturgeons, etc.), the posterior 
limb remains the same, while the anterior limbs have undergone ` 
a great abbreviation in the loss of the axial bones and the reduc- 
tion of the number and length of the actinosts. In the third 
group, or Actinopteri,? both limbs have undergone reduction, the 
actinosts in the posterior fin being almost all atrophied, while 
those of the fore limb are much reduced in number. 
The phylogeny of these tribes is not easy to make out at pres- 
ent. The descent has been no doubt in the order named in time, 
but the starting point is yet uncertain. Thus the Chondrostei 
appear later in time than either of the other tribes, a history which 
probably only represents our ignorance. The characters of the 
genus Crossopholis Cope, from the American Eocene, strongly 
suggests that the existing forms have descended from scaled an- 
cestors. The Crossopterygian fore limb, with its arm-like axis, 
tells of the origin of the first limbed vertebrates, the Batrachia, 
whose skull-structure, however, only permits their derivation 
from the Dipnoi or Holocephali. As the former subclass has the 
Crossopterygian fin structure, we can safely regard them as the 
ancestors of the Batrachia, while the Crossopterygia are a side 
line from a similar type, probably the Ichthyotomi, because these 
have a free suspensor of the lower jaw. But of the structure of 
the fins of the Ichthyotomi unfortunately we know nothing. If 
this position be true, then the successive derivation of the Chon- 
drostei and the Hyopomata in one line is rendered probable. The 
modification of structure has consisted in the contraction of the 
supporting elements of the pectoral and ventral fins by the reduc- 
tion of their numbers and length. According to paleontological 
1 Proceedings American Assoc. Adv. Sci., 1884. 
3 Partly agrees with the Teleostei of Müller, but includes many of his Ganoidea. 
