70 S. W. Williston. 
either group are of comparatively trivial importance; the remarkable 
elongation of the vertebral spines in the more specialized pelycosaurs 
is of scarcely greater moment than are similar modifications among 
some existing lizards. ‘The presence of the cleithrum, the old-fash- 
ioned character of the girdles in general, and the primitive struc- 
ture ot the feet have been recognized in the pelycosaurs, but it has 
been supposed that radical differences existed in the skull, differences 
sufficient to separate the group not only ordinally but into a distinct 
sub-class. My present studies of the skull of Labidosaurus, one of 
the most primitive of the reptiles known from the Permian, prove, 
satisfactorily to myself at least, not only that there are close morpho- 
logical resemblances between the skeletons in these two groups of rep- 
tiles, but that these resemblances extend to the skull as well; that we 
have in Labidosaurus and its allies, the Pariotichide, a persistence 
of those generalized characters which gave origin to the peculiar 
specializations of the pelycosaurs; that, in other words, the pelyco- 
saurs are simply an immediate branch from the primitive cotylosaurs, 
having no direct affinities with the Rhynchocephalia; that their rela- 
tionships are, probably, far closer with the ichthyosaurs and the 
proganosaurs. I am aware that McGregor, in his latest studies of 
the Proganosauria,’ still adheres to the belief that the group is closely 
allied to the rhynchocephalian stem, a member not only of the “Diap- 
sida” but also of the ‘‘Diaptosauria.” 
His belief necessarily assumes 
the presence of two temporal vacuities in Stereosternum and Meso- 
saurus, Which has not been proven and of which I am very skeptical. 
Moodie recently has ventured the suggestion that the reptiles have 
arisen exclusively from the Microsauria and not from the Temno- 
spondyli, but, until he finds better arguments to sustain his conten- 
tion than he has so far furnished, I doubt whether his opinions will 
obtain acceptance. To evolve five-toed reptiles from four-toed 
amphibians; to develop a vestigial cleithrum in such forms as 
Dimetrodon, Diadectes, Pareiasaurus, Dicynodon, ete., from forms 
which did not possess it; to originate a vestigial intercentrum in rep- 
1Commissao de Estudos das Minas de Carvao de Pedra do Brazil, II, p. 
301, 1908. 
“Geological Magazine, vol. vi, 1909, p. 216. 
