396 THE ANATOMY OF YERTEBEATED ANIMALS. 



The occipital condyles are, consequently, situated at tlie pos« 

 tenor end of the basis cranii, and the basi-cranial axis is as 

 long as the cerebral cavity. In Chrysothrix^ on the contrary, 

 the face is relatively small, with a high facial angle ; the brain- 

 case is moderately arched ; the plane of the tentorium is hori- 

 zontal, like that of the occipital foramen, which lies but little 

 behind the middle of the base of the skull. The basi-cranial 

 axis is much shorter than the cerebral cavity. The j)i'6- 

 maxillo-maxillary suture disappears early in Cebus. 



The formula of the adult dentition is i. ~. c. ~ p.m. |^ 



m^ il|=:36. The crowns of the molar teeth usually have tw^o 



transverse ridges, ending in four cusps. In the upper molars 

 of Ateles and Mycetes an oblique ridge crosses the crown from 

 the antero-external to the postero-internal cusp. The perma- 

 nent canines usually make their appearance before the last 

 molar. 



The stomach is simple, the caecum large, and devoid of 

 any vermiform appendix ; the liver is usually five-lobed ; and 

 the kidney has a single papilla. 



The ventricles of the larynx are not usually developed into 

 air-sacs. In Ateles^ however, a median air-sac is developed 

 from the posterior wall of the wdndpipe between the cricoid 

 cartilage and the first ring of the trachea. A very remarkable 

 modification of the hyoid and larynx takes place in Mycetes. 

 The cornua of the hyoid are rudimentary, but its body is con- 

 verted into a large thin-walled bony drum, the cavity of which 

 communicates, beneath the large epiglottis, with that of the 

 larynx. The thyroid cartilage is very large, and the carti- 

 lages of Wrisberg and Santorini are replaced by a fibrous mass, 

 which is united posteriorly with its fellow^ of the opposite side. 

 In addition to the hyoidean air-sac the ventricles of the larynx 

 are dilated and prolonged upward, coming into contact above 

 the larynx ; two pharyngo-laryngeal pouches may be added 

 to these. Mycetes is famous for the distance to which its 

 howling voice can be heard in the South American forests. 



Although the pollex is rudimentary and apparently func- 

 tionless in Ateles^ all its characteristic muscles (abductor^ ad* 

 ductor^ flexor brevis, and op2:)onens) are present, except the 

 long flexor. 



In Nyctipitheciis the pedal inter ossei are flexores breves, 

 and lie on the plantar surfaces of the metatarsal bones, as in 

 the Marmosets ; but both the adductor Jiallucis and the trans* 

 versus pedis are well developed. 



