1 86 Bulletin American Museum of Natural History. [Vol. XVI, 



Skull of Hyopsodus. — The most beautiful specimen in the 

 collection consists of a skull and jaws of Hyopsodus (Fig. 5), 

 collected during the American Museum expedition of 1895 

 by Dr. J. L. Wortman. Unfortunately the anterior region of 

 the orbit is fractured, but on the left side the lachrymal 

 foramen is seen to be marginal or internal as in the Anthro- 

 poidea and not external as in many of the Lemuroidea; the 

 infraorbital foramen is placed above the interval between the 

 third and fourth premolars ; the temporal fossa is surmounted 

 by a thin sagittal crest, which is broken away in this specimen. 

 The external auditory meatus is widely open inferiorly, and 

 there is no trace of a tympanic tube (the absence of a tym- 

 panic tube distinguishes the South American from the 

 European monkeys) ; the bulla is also broken away, exposing 

 a portion of the semicircular canals. Behind the auditory 

 meatus is apparently a narrow exposure of the mastoid, per- 

 forated by the mastoid foramen; the posterior nares open 

 just behind the last molar. 



Dentition. — The last molar is a small tooth with a 

 small hypocone, the first and second molars have the 

 hypocone better developed. Unlike H. niarshi, the third and 

 fourth premolars only have internal cusps (deuterocones), 

 the second premolar has an internal basal cingulum, the first 

 is a simple conical tooth; close in front of this is the canine, 

 a much larger tooth. The grinding series do not converge 

 anteriorly, being nearly parallel; this is an important point. 

 There is a faint external cingulum. 



The lower jaw. — The inferior molars measure 14 mm; 

 they cannot be distinguished from those of H. paulus ; the 

 animal was thus a small one. The fourth lower premolar 

 presents a deuteroconid, the third premolar presents a rudi- 

 ment of the same, the second and first are absolutely simple. 

 The jaw has a well marked masseteric fossa, the condyle is 

 raised somewhat above the level of the molar series, the chin 

 is not very distinctly defined, the mandibular symphysis is 

 coossified. 



(Sp. 29) Hyopsodus minusculus Leidy. — This appears to be 



