, Explanation of Plate YI 



Jaw muscles of monimostylic reptiles: C'lielydra, Sphcnodoii, iiud Alligator. 

 The pretrematic muscles (iuuervated by V3) are seen in front of the audi- 

 tory region, the posttrematic muscles (innervated by YII) are seen 

 behind it 



Figs. 1 and 2. — Chelydra. 



The deep muscles are shown in Fig. 1, and the supertit;ial in Fig. 2. 



The capiti-uiandibularis (Cm.) has penetrated the dermal skull roof and 

 after passing through the posttemporal opening is attached to the long sagittal 

 crest. The pterygoideus anterior runs obliquely across the capiti-mandibularis 

 (Cm. s.; Cm. in.). These two muscles coopei'ate in closing the jaw. The 

 depressor mandibulte {D. m.) arises from the posterior border of the squamosal 

 behind the auditory region and is inserted into the posterior process of the 

 mandible. The extreme simplicity of the jaw muscles in the Chelonia is asso- 

 ciated with the rigid fixation of the upper jaw. 



Figs. 3 and 4. — Sphenodon. (Mainly after Osawa. ) 



The superficial muscles are shown in Fig. 3, and the deep muscles in Fig. 4. 



The fibres of the superficial (masseter) layer of the capiti-mandibularis 

 (Cm.s.) cross those of the middle layer nearly at right angles. The middle 

 layer {Cm.m.) arises around the borders of the supratemporal fenestra. The 

 lateral temporal fenestra affords spac*> for the expansion of this muscle. The 

 deep layer ( C m. p. ) arises from the lower borders of the supratemporal 

 fenestra and from the side of the brain-case in the auditory region. The 

 pterygoideus anterior {I't.ant.) is inserted around the lower posterior border 

 of the mandible. 



Figs. 5 and 6. — Alligator. 



The capiti-mandibularis ( C m. s. ; C ni. m. ) is inserted into the Meckelian 

 fossa of the mandible. The middle slip runs up to the small supratemporal 

 fenestra. The pterygoideus anterior {Ft. ant.) is extended forward beneath 

 and in front of the oj-bit. Tlu> pterygoideus posterior {Pt. post.) wraps around 

 the posterior end of the mandible. 



