72 



PROCEEDINGS OF THE NATIONAL MUSEUM 



size and general structure the fossil skull displays many resemblances 

 to the living lizard Crotaphytus. Tlie dentitions of these two forms 

 likewise are very similar. 



Viewed from the side (see fig. 31) the profile of the skull at the 

 junction of the parietal and frontal is depressed, as contrasted with 

 the usual convex profile of most of the Iguanidae. From the tip of 

 the nose to the posterior end of the squamosal the skull has a greatest 

 length of 27 mm. ; the greatest breadth across the jugals is 14.6 mm. 



The premaxillary has a long spine that is relatively wider than in 

 Crotaphytus. Its posterior end is notably different in being broadly 

 rounded as contrasted with the narrow, sharply pointed extremity 

 in the extant genus. The nasals are short and wide, being shortened 



Figure 30. — Skull of Aciprion formosum Cope (U.S.N.M. No. 16566), superior view: /, 

 Frontal; ju, jugal; la, lachrymal; mx, maxillary; na, nasal; p, parietal; pmx, premaxil- 

 lary; pof, postorbital; prf, prefrontal; soc, supraocciptal; sg, squamosal. About three 

 times natural size. 



by the large size and partly vertical position of the nostril openings. 

 The frontal is single and relatively wide between the orbits. The 

 pineal foramen is on the frontoparietal suture. The prefrontal is 

 large, but without a preocular boss, which forms such a prominent 

 projection on the Crotaphytus skull. The postfrontal is absent, a 

 condition noted by Cope^ in Crotaphytus. Its place is taken by a 

 widening of the frontal on each posterior-external angle. The 

 postorbital is large, uniting inferiorly with the jugal and posteriorly 

 with the squamosal. The dorsal surface of the parietal is relatively 

 narrower between the supratemporal fossa and between the divergent 

 posterior process than in Crotaphytus. The left squamosal is miss- 

 ing, and only a small part of the right one is present. In the illustra- 



1 Cope, E. D., Ann. Rep. U. S. Nat. Mus. for 1898, p. 246, 1900. 



