ARCHCEOHIPPUS. 



211 



GENUS ARCILffiOHIPPUS Gidley 1906. 

 Bull. Amer. Mus. Nat. Hist. Vol. XXII, Art. XXII, Dec. 19, 1901), pp. 385-388. 



Genotype. — Archceohippus (Anchitherium) ultimus Cope, from the Mascall, Middle Miocene, Ticholeptus zone of 

 Cottonwood Creek, Oregon. This genus is represented by only two species at present, A. ultimus and A. mourningi, 

 from the Middle Miocene of Oregon and Upper Miocene of California, and is known only from the anterior portion of a 

 skull and the grinding teeth. 



The chief characters are the following: 



1. Grinding teeth short-crowned, protoloph and metaloph continuous, protocone partly distinct, sharply crested 

 cones and conules, purely lophoid, not bunoid. 



2. Internal cingulum vestigial or wanting. 



3. Crochet absent, hypostyle undeveloped. 



4. M| not reduced. 



5. Facial region relatively longer than in Hypohippus, orbits placed farther back. 



6. Preorbital laehrymo-malar fossa very deep. 



7. Anterior border of orbit above m 3 . 



8. Incisor teeth small. 



9. "P4 probably not larger than mi, as indicated by the comparatively large upper true molars." (Gidley, 1907, 

 p. 80S.) 



TICHOLEPTUS-MERYCHIPPUS RELICTUS ZONE. 9. EARLY MIDDLE PLIOCENE. 



This zone is typified by the Mascall of Oregon. 



Archaeohippus ultimus Cope 1886. 

 Plate 6.2. Text Fig. 172. 



Anchitherium ullimum, sp. nov., Cope, E. D. "On Two New Species of Three-toed Horses from the Upper Miocene, with Notes 

 on the Fauna of the Ticholeptus Beds," Proc. Amer. Philos. Soc. Vol. XXIII, April 15, 1886, pp. 357-358, no figure. Archceohippus 

 ultimus (Cope), Gidley, J. W., "A New Genus of Horse from the Mascall Beds, with Notes on a Small Collection of Equine Teeth in the 

 University of California," Bull. Amer. Mus. Nat. Hist. Vol. XXII, Art. XXII, Dee. 19, 1906, pp. 385-388, figs. A, B. 



Horizon and locality. — Middle Miocene, Mascall formation, Cottonwood Creek, Ore., Ticholeptus zone. Type 

 collected by J. U. Wortman. 



Type. — Amer. Mus. Cope Coll. 8174. Anterior portion of a skull, with nearly complete dentition. Measurements: 

 (Cope) superior molar series .079; true molars m 1-3 .034; m 1 a.p. .011, tr. .015; m 3 a.p. .011, tr. .014. 



Type figure. — Plate 6.2, text Fig. 172 of this Memoir. 



Characters. — (Cope, 1886, Osborn, 1918) (1) Size diminutive. (2) Dental series of same length as that of Miohippus 

 longicristis; (3) premolars and molars with well marked external cingulum and internal cingulum around base of p 2 ; (4) 

 weak cingulum around protolophs of m 2-3 ; (5) long diastema between canine and p 1 . (6) A deep and large preorbital fossa 

 separated from the orbit by a vertical ridge; (7) anterior border of orbit above anterior border of m 3 , agreeing only with 

 Kalobatippus prastans; (8) antorbital foramen above middle of fourth premolar; (9) lateral narial opening extending very 

 far back. (Gidley, 1906, p. 386) (10) Complete union of metaloph with ectoloph; (11) proportionately larger size of 

 protoconule; (12) crochet on metaloph absent or slightly developed; (13) comparatively equal proportions of molars and 

 premolars; (14) presence of all developed internal basal cingulum. (15) Elongation of preorbital region of the skull; 

 (16) backward position of the- orbit; (17) great development of lachrymo-malar fossa?. 



