1s | 
The affinities of the Pelycosauria. 
There can not be any doubt that Dimetrodon is nearest to 
the Rhynchocephalia and Proganosauria (Palaeohatteriidae). 
The structure of the skull, the vertebral column, and the humerus 
are of the same type. The presence of a distinct squamosal and 
prosquamosal is of special interest. The same condition we find 
in Sapheosaurus H. v. Meyer (Sauranodon JourDAN) of the 
Jurassic Sapheosauridae !); and there is very little doubt that these 
two elements are also present in Palaeohatteria CrepNER. The 
bone marked squamosal by CREDNER ?) is the prosquamosal; the true 
squamosal must have been free, and connected with the parietal pro- 
cesses. 
In Sphaenodon the maxillary forms the lower boundary of the 
orbit; in Palaeohatteria and Dimetrodon, the jugal excludes 
the maxillary from the orbit. The vertebrae with the well-developed 
intercentra, the ribs with the double articulations, can only be com- 
pared with those of the Rhynchocephalia and Proganosauria 
(Palaeohatteriidae). The presence of a free central bone in 
the tarsus of the Pelycosauria is an original character, which is 
shared only by the Palaeohatteriidae and Proterosauridae; 
but in the Palaeohatteriidae tarsals 4 and 5 are free, in Pro- 
terosaurus and Dimetrodon they are united, to support meta- 
tarsal 4 and metatarsal 5. The humerus of Dimetrodon can be 
directly reduced to that of Sphaenodon. The entepicondylar foramen 
is well-developed in both, the ectepicondylar foramen of Sphaenodon 
is represented by a very distinet ectepicondylar groove in Dime- 
trodon. 
The specialization of the Pelycosauria consists in the enor- 
mous development of the neural spines of the dorsal vertebrae, and 
in the reduction of the upper part of the quadrate and its nearly com- 
plete inclosure by the squamosal, prosquamosal and quadratojugal. It 
is quite evident, that the Pelycosauria with the two 
temporal arches, and the specialized neural spines can 
not be the ancestors of Mammals; they represent a specialized 
side branch of a line leading from the Proganosauria to the Rhyncho- 
cephalia, which becomes extinct in the Permian. 
1) G. Baur, Bemerkungen über die Osteologie der Schläfengegend 
der höheren Wirbeltiere. Anat. Anz., Bd. X, 1894, p. 321—322. 
2) HERMANN Crepner, Die Stegocephalen und Saurier aus dem Rot- 
liegenden des Plauenschen Grundes bei Dresden. Zeitschr. d. Deutsch. 
Geol. Ges., 1888, Pl. XXXV, Fig. 1. 3. 4. 
