ADAMS, PHYLOGENY OF THE JAW MUSCLES HI 



rangement, as their jaw action is primarily a straight opening and closing 

 movement, either a hard, steady pull, or with a snap. The opposite halves 

 of the mandible are usually strongly fastened together at the symphysis. 

 The condyle is lengthened out laterally toward the median line and the 

 glenoid cavity has a large protecting shelf to give a firm hold on the 

 condyle. In fact in some of the carnivorous types with strong jaws the 

 articulation is locked, so that the borders of the glenoid cavity must be 

 broken in order to separate the mandible from the skull (Gulo luscus). 

 With the shearing teeth and the locked type of articulation for the con- 

 dyles of the jaw the pterygoid muscles are of little value and in carniv- 

 orous forms are always weak, as in Canis, Gulo, Lutra, Didelphys. In 

 these animals the basicranial region of the skull shows the reduction of 

 the pterygoid muscles. 



MONOTREMES 



Plate IX, Figs. 1-7 



Skull. — The skull and jaws in both Ornithorhynchus and Echidna are 

 of very aberrant and more or less degenerate types. In both genera, also, 

 the pterygoid region is highly modified, and thus the homology of the 

 so-called pterygoid elements is open to question. Ornithorynchus re- 

 quires fairly strong jaws for the crushing of small mollusc shells, while 

 Echidna has practically vestigial jaws and depends mostly on the tongue 

 muscles. 



The condition of the jaw musculature in the monotremes is quite sug- 

 gestive of the reptiles. The massed condition of the temporal-masseter 

 group suggest the condition of the capiti-mandibularis of the reptiles. 

 The head muscles as a whole seem to be homologous with those of other 

 mammals, especially as regards the temporalis-masseter, pterygoideus 

 externus and muscles of the ventral hyoid group, with the exception of the 

 anterior belly of the missing digastric, which may be represented by the 

 muscle called depressor mandibular anterior. However, this hyoid region 

 in the monotremes is very specialized, so that the derivation of the an- 

 terior belly from this region is rather obscure, although some anatomists 

 hold this opinion. It is probably a slip from the mylohyoid. The duck- 

 like bill of the Ornithorhynchus requires a special musculature which is 

 developed from the V 3 muscles. 



The monotremes are so different from other animals in many important 

 details of their myology that it is probably correct to assume that they 

 split off from the mammalian stem at a very early period. This assump- 

 tion is strengthened by the fact that the osteology and soft anatomy are 



