. Carnegie Institution of Washington publication. MORPHOLOGICAL REVISION OF THE SUBORDER. 85 Atlas: The centrum of the atlas is lost, but the neural arch of one side clings to the under side of the skull; it is \^xy similar to the neural arch of Dimetrodon (plate 16, figs. 3 and 4). Axis: The centrum of the axis is somewhat elongate and the bottom line forms a sharp ridge. The sides are concave and marked below the transverse process by several longitudinal lines. Just below the transverse process of the left side is a good- sized nutrient foramen. The transverse process springs from the neura
. Carnegie Institution of Washington publication. MORPHOLOGICAL REVISION OF THE SUBORDER. 85 Atlas: The centrum of the atlas is lost, but the neural arch of one side clings to the under side of the skull; it is \^xy similar to the neural arch of Dimetrodon (plate 16, figs. 3 and 4). Axis: The centrum of the axis is somewhat elongate and the bottom line forms a sharp ridge. The sides are concave and marked below the transverse process by several longitudinal lines. Just below the transverse process of the left side is a good- sized nutrient foramen. The transverse process springs from the neural arch and is very short. The articular face is small and the distal end of the process is not free from the side of the centrum. The spine is rather high, thin, and expanded antero- posteriorly, with a wide superior edge. The anterior zygapophyses are small, but not so disproportionately small as in the ClepsydropidcB. Centrosphene and centantrum (see morphological description of Dimetrodon incisivus) are present, but small. Tkiid vertebra (counting the atlas as first): The bottom line of centrum has a distinct low keel, but the longitudinal lines on the sides are less definite. The dia- pophysis is longer than in the axis and inclined more sharply to the rear. The lower portion of the anterior face of the centrum is slightly cut out to accommodate an inter- centrum. The spine is of nearly the same shape as in the axis. Several small two- headed ribs, which probably belong to the anterior cervicals, cling to the side of the skull. Fourth to seventh vertebrcB: In these the diapophyses grow gradually shorter and stand out more nearly straight from the centrum. They do not originate, as in Dimetrodon^ in three processes, one each from the anterior and posterior zygapophysis and one from below, but rise directly from the neural arch below the line of the zyga- pophyses. An inferior supporting ridge is present, which at first inclines backward, but soon changes so that on the fifth
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