. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Middle Penxsylvaxian Pelycosaltis • Reisz lar situation exists but the anterior teeth are larger tlian those behind them, while in Echinerpeton these teeth are not strongly differentiated. At the posterior end of the dentar)', the teeth are very small. Axial skeleton. The incomplete nature and disarticulation of the type specimen, MCZ 4090, makes determination of the exact number of presacral \ertebrae impos- sible. Partial reconstruction of the skele- ton has been attempted, however (Fig. 11). On the basis of this re
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Middle Penxsylvaxian Pelycosaltis • Reisz lar situation exists but the anterior teeth are larger tlian those behind them, while in Echinerpeton these teeth are not strongly differentiated. At the posterior end of the dentar)', the teeth are very small. Axial skeleton. The incomplete nature and disarticulation of the type specimen, MCZ 4090, makes determination of the exact number of presacral \ertebrae impos- sible. Partial reconstruction of the skele- ton has been attempted, however (Fig. 11). On the basis of this reconstruction there must ha\e been at least 23 presacral \erte- brae. Since the t\pical number of presacrals in the great majority of pelycosaurs is 27, it is probable that at least four are missing in this specimen. The vertebrae in the ante- rior portion of the column are found in close association with each other but are not articulated, and the centra have sepa- rated from their neural arches. The mid- dorsal and anterior dorsal \ertebrae are found scattered all o\er the block. There is also some \ertebral material of a \'ery similar nature in blocks C and D. The centra are not elongated. In the cenical and anterior dorsal regions they are 5 mm long and 4 mm high at the poste- rior rim. The centra in the mid-dorsal and posterior dorsal x'ertebrae are about I'cjual in length and height. In the primitive romeriid Hylonomus and in Archaeotlujris, the centra are more elongated. In later ophiacodonts, howe\-er, the centra tend to be compressed; this shortening of the cen- trum is most strongK- marked in Ophkico- don retroverstis. The keel de\elopment. more prominent in the anterior region of the column, ne\er reaches the levels found in ad\-anced sphenacodonts in which promi- nent \entral keels are present and the centra have strongly excavated lateral margins. In Echinerpeton the \entral lip of the centra is not strongly bevelled for the re- ception of the intercentra, indicat
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