- 
Reviews—Dr. van Hoepen—Stegocephalia, Senekal. 85 
In the reviewer's opinion the evolution in the large amphibia 
took the following course. ‘The amphibia were derived from 
unknown ‘Crossopterygian’ fish which, like all known members of 
that group, had a trunk of circular section and a somewhat depressed 
snout. The skull with the lower jaw formed a mass as high as wide 
.at the neck, asin Maw’s uncrushed ‘ Zoxvomma’ skull.. The body was 
long (more than twenty-nine presacrals in Pteroplax ?), and the tail 
probably even longer ( Pholidogaster and Cricotus). The animals, as 
shown by the grooves for lateral line organs on the skull, were mainly 
aquatic. In Lower Permian times, whilst retaining their round body, 
they became largely terrestrial (Hryops and especially Cacops and 
allies), the lateral line grooves becoming obscure or absent. 
Subsequently, for some cause of whose nature I can make no 
suggestion, the head and anterior part of the body began to become 
depressed, the process culminating in such extraordinarily flattened 
forms as Cyclotosaurus. Concurrently the animal became secondarily 
aquatic. Many changes in the skull are correlated with this 
depression ; of these the most important are: (1) The gradual dorso- 
ventral thinning of the basiscranii, which leads to the almost complete 
suppression of the basi-occipital and basisphenoid, to the replacement 
of the single basi-occipital condyle of the early forms by the paired 
exoccipital condyles of later types, a process in which many stages are 
now known, and to the gradual replacement of stout basipterygoid 
processes of the basisphenoid by thin but necessarily broader 
expansions of the parasphenoid. (2) The gradual expansion of the 
interpterygoid vacuities from small slits in the Carboniferous forms to 
the enormous openings in Cyclotosaurus. Not so obviously correlated 
with the depression of the skull is a gradual shortening of that part 
of the pterygoids and of the quadrates, squamosal, etc., connected 
with them, which lie behind their articulation with the basiscranii. 
This results in the exoccipital condyles forming the extreme posterior 
‘points of the skull in many Triassic types. 
‘The primitive Carboniferous amphibia had an embolomerous column 
in which the pleurocentra and intercentra are perforated discs of 
nearly equal size; reduction of the upper part of the intercentra and 
of the lower part of the pleurocentra leads easily to the rachitomous 
type found in Permian forms: still further reduction of the pleuro- 
centra with a concurrent strengthening of the intercentra leads to the 
stereospondylous column found in the Triassic animals (there is 
unsatisfactory evidence in the Stuttgart Museum suggesting the 
occurrence of small pleurocentra in Mastodonsaurus and DMetopo- 
saurus). The clavicles of the typical Carboniferous embolomerous 
Pteroplaz are flat plates with parallel anterior and posterior margins 
very like those of the fish Megalichthys. he interclavicle is a very 
small rhomboidal bone with a rudimentary posterior stern. In the 
more terrestrial Lower Permian types, e.g. Hryops and Cacops, the 
upper end of the clavicle is narrowed and affixed to the front edge of 
the scapula, separated from it, however, by the cleithrum, and the 
interclavicle becomes proportionately larger. In the Triassic forms, 
in correlation with the depressed form of the head and anterior part of 
the body and probably with the aquatic habits, the lower end of the 
