The reptiles and birds, which we group together as the Sauropsids, generally agree in the characteristic structure of the skull and brain, and this is notably different from that of the Mammals. Further, in the Sauropsids the skin is clothed with scales or feathers; in the Mammals with hair. The red blood-cells of the former have a nucleus; those of the latter have not.
Homoeosaurus pulchellus, a Jurassic proreptile from Kehlheim. The instructive group of the Permian Tocosauria, the common root from which the divergent stems of the Sauropsids and mammals have issued, merits our particular attention as the stem-group of all the Amniotes.
Hence it has not yet been possible to arrange these important fossils with any confidence in the ancestral series that descend from the Protamniotes to the Sauropsids on the one side and the Mammals on the other. Opinions are particularly divided as to the place in classification and the phylogenetic significance of the remarkable Theromorpha.
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