1885.] Progressive and Retrogressive. 243 
The axial elements (femora) of the posterior fins unite together, 
become very. elongate and lose the natatory portion. In one 
group (Orthagoriscidz) the posterior part of the vertebral col- 
umn is lost and the caudal fin is a nearly useless rudiment. In 
‘the Ostraciontide (which may have had a different origin as the 
pharyngeal bones are not contracted), the natatory powers are 
much reduced, and the body is enclosed in an osseous carapace 
so as to be capable of very little movement. The entire order is 
deficient in osseous tissue, the bones being thin and weak. -Itis 
a marked case of degeneracy. 
There are several evident instances of sporadic degeneracy in 
other orders. One of these is the case of the family of the Icos- 
teidz, fishes from deep waters off the coast of California. Al- 
though members of the Percomorphi, the skeleton in the two 
genera Icosteus and Icichthys is unossified, and is perfectly flexi- 
ble. Approximations to this state of things are seen in the para- 
sitic genus Cyclopterus, and in the ribbon fishes, Trachypteride. 
Thus nearly all the main lines of the Physoclysti are degen- 
erate; the exceptions are those that terminate in the Scombride | 
(mackerel), Serranida, and Scaridz (Pharyngognathi), 
V. THE LINE OF THE BATRACHIA. 
We know Batrachia first in the coal measures. They reach a 
great development in the Permian epoch, and are represented by 
large species in the Triassic period. From that time they dimin- 
ish in numbers, and at the present day form an insignificant part 
of the vertebrate fauna of the earth. The history of their suc- 
cession is told by a table of classification, such as I give below: 
I. Supraoccipital, intercalary and supratemporal bones present. Propodial 
n . 
Vertebral centra, including atlas, segmented, one set of segments together support- 
ing one arch......ssseserseesesroeseosocortaecece:osnpaesopo achitomi. 
_Vertebree segmented, es superior and inferior segments each bömplete, forming two 
centra to each arc 
Vertebral centra, cas atlas, not deginensed; one to each arch.,... Speen. 
II. Supraoccipital and supratemporal bones wanting. Frontal and propodial 
nes disti 
a. An os intercalare. 
A palatine arch and separate caudal vertebre Proteida. 
aa, No os intercalare. 
