MORPHOLOGICAL REVISION OF THE SUBORDER. 



Family POLIOSAURIDAE Case (seep. i8).* 



Genus POLIOSAURUS Case (see p. i8). 



The discussion of the genus is contained in that of the single species. 

 Pollosaurus uniformis Cope (see p. 19). 



Characteristic specimen, No. 1148, the type (plate i, figs. 1-8). 



The original characters used by Cope have proven to be of little value, but the 

 ones listed in the redescription of the genus show that the animal occupied a most 

 important position in the order. As already shown by the author (29) Poliosaitriis is 

 probably the most primitive member of the group ; the nearest to the ancestral form. 



Only the preorbital portion of tlie skull is preserved and it is so badly crushed 

 that its original form is not easily made out, but the nasals and frontals are broad, 

 showing that the top was flattened acuminate. The orbits were large and lateral, and 

 the nares small and anterior in position. Cope described large teeth in the premax- 

 illary and in the anterior end of the dentary. The only evidence of them is a single 

 large, detached tooth which looks as if it had belonged in the anterior end of the 

 lower jaw. Cope also describes a diastema between the maxillary and prema.xillary ; 

 of this there is no trace. From the anterior end of the series the teeth gradually 

 increase in size until near the middle and then as gradually decrease. The largest 

 are three times the size of those at the anterior and posterior ends. 



The vertebrse regarded by Cope as belonging to the genus PoUosaiiriis are all 

 characteristically shortened antero-posteriorly, so that the centra are nearly as wide as 

 long ; the articular faces of the centra are reverted on the sides and the arches are free 

 from the centra. The shape of the articular face on the centrum for the neural arch 

 slants gradually to the rear, growing deeper, and then rises almost vertically (see 

 plate I, figs. 6, 9, 11), leaving a deep triangular pit. The two faces are divided by 

 the neural canal which is much deeper in the middle than at either end, so that it is 

 also a deep triangular pit. Cope evidently used this character of a free neural arch 

 and the shape of the articular faces as decisive characters in determining the speci- 

 mens labeled by him, but they are indeterminate, as in young specimens of the genera 

 Dimetrodon and Clepsydrops the neural arch is frequently free, especially in the 

 caudals, and their articular faces have the same characters as in Poliosaiirus. 



There are seven vertebrae in connected series and si.x more unattached, but evi- 

 dently belonging next to those preserved. 



The atlas is illy preserved so that its form can not be made out; the neural arches 

 were free and have been lost. It is evident that the form of the atlas was nuich as in 

 Dimetrodon (see plate 16, figs. 5, 6). The remnants of an atlantal rib cling to the 

 sides of the centrum. 



The axis is rather elongate, nearly twice as long as the atlas. The centrum has 

 no keel, but there is a sharp, slender ridge along the mid-line of the lower surface. 

 The anterior and posterior articular faces are only slightly reverted on the sides of the 

 centrum. The spine is short, elongate antero-posteriorly and proportionately very 

 heavy. The posterior zygapophyses are large, the anterior ones are very small, 



*Page references after names refer to Systematic Revision. 77 



