74 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 91 



median and two small lateral cusps. These lateral cusps are most 

 prominently developed on the teeth of the posterior two-thirds of 

 both upper and lower series. From this point forward the teeth 

 gradually diminish in size, and the lateral cusps become smaller and 

 smaller, disappearing altogether on the first few teeth that have simple 

 pointed crowns. Upper and lower teeth appear indistinguishable. 

 Crowns in lower jaw project farther above the alveolar border than 

 in the maxillary. 



Specimen U.S.N.M. No. 16566 in total number of teeth in maxillary 

 and dentary is in perfect accord with the type of Acip7ion majus 

 Gilmore, but its smaller size clearly shows it to pertain to the earlier 

 described Aciprion foniwsmn Cope. 



Rennarhs. — In 1928 ^ this genus was referred to the family 

 Iguanidae on rather meager evidence, but after a study of these new 

 materials the propriety of that assignment now seems assured. The 

 resemblances found in skull structure and character of dentition to 

 those of extant members of the family leave little doubt as to the 

 correctness of this family assignment. 



Measurements of Skull, U. 8. N. M. No. 16566 



mnu 



Greatest length of skull, over all 27.0 



Greatest length of skull at middle 22.3 



Greatest width of skull across jugals 14.6 



Greatest width parietals at center 3.5 



Greatest length frontals between orbits 2.3 



Greatest length nasal 3.6 



Greatest length frontal 7.0 



Greatest length parietal 7.1 



Greatest width occipital condyle 1.2 



Genus EXOSTINUS Cope 



EXOSTINUS SERRATUS Cope 

 FiGUBE 32 



An anterior portion of a skull and a left dentary (U.S.N.M. No. 

 16565) is clearly identified as pertaining to Exostinus serratus Cope. 

 It is the first specimen found that displays the complete structure and 

 osseous scutellation of this part of the cranium, and thus it contributes 

 to a better understanding of this little-known species. 



The entire outer surfaces of the premaxillary, nasal, and maxil- 

 lary bones, with the exception of a smooth narrow band parallel to 

 the dentigerous border, is covered by the characteristic osseous promi- 

 nences, as shown in figure 32. These are coalesced to the underlying 

 skull elements and thus hide all trace of the cranial sutures. For that 



'Gilmore, C. W., Mem. Nat. Acad. Soi., vol. 22, p. 18, 1928. 



