REPTILIA : SPHENACODON 79 



with the same individual as did the dentary and may be considered 

 as part of the type. These were the only specimens in the lot 

 studied by Marsh. Later finds by Baldwin show several other 

 mandibles and maxillae, but the numerous pieces into which they 

 were broken were widely scattered through the collection. They 

 were, if seen at all by Marsh, probably not attentively examined. 

 It is surprising, however, that in an entire collection from this 

 horizon, and this bone-bed, a collection including perhaps hundreds 

 of vertebrae, I can find no indications of long, Dimetrodon-lihe spines. 

 Marsh stated that the vertebrae are deeply biconcave, but the 

 fact is that not a single vertebra can be certainly correlated with the 

 type specimen or species. There are twenty-four teeth or sockets 

 for teeth in the dentary, a number somewhat under the usual one 

 in Dimetrodon. The first and third, as preserved, are of nearly 

 equal size; they are longer than the ones following, are more 

 pointed and convex above. The second tooth in life was probably 

 twice the length. The remaining teeth are of nearly equal size, 

 save the last four, the roots of v^^hich are distinctly smaller. The 

 fourth and the sixth have each a distinctly worn facet on the outer 

 side, which fits precisely the outer facets of the caniniform 

 tooth of the corresponding maxilla. In the left maxilla (Plate 

 XXXVII, Fig. i), there are two smaller teeth in front of the very 

 large caniniform tooth. Back of this tooth there is a cavity on 

 each side, apparently for a tooth of considerable size. This con- 

 dition is noted by Case as characteristic of the Clepsydropidae, 

 with rare exceptions; in another dentary of smaller size in the 

 collection the alveolus is filled, the three large teeth being all nearly 

 of one size. Back of this cavity there are twelve teeth or cavities 

 for teeth. They are all of a slightly larger size than those in the 

 dentary, and, with the caniniform tooth biting in the place shown 

 by the worn facets, the teeth would extend back of the lower ones 

 about fifteen millimeters. The whole number in the maxilla was 

 but sixteen, a smaller number than is known in species of Dimetro- 

 don. None of the teeth is serrated on its margins. It is evi- 

 dent from the position of the anterior process in front of the first 

 tooth that the border of the diastema was not ascending. The 

 remainder of the collection obtained later by Baldwin, evidently 



