Explanation of Plate IX 



Jaw muscles of monotremes: Oniithorhynclius, Echidna. After Schulinan 



The jaw muscles of monotremes as well as their skulls exhibit peculiar 

 specializations along with certain very ancient characters not found in other 

 mammals. 



Fig. 1. — Omithorhynchus. Superficial muscles. (Muscles mainly after Schul- 

 man. ) . 



The capiti-mandibularis of reptiles is now represented by the masseter 

 (Mas.), temporalis (Tern.), and pterygoideus externus. The masseter is sub- 

 divided into several muscles of which the detrahens mandibular (Det.m.) 

 simulates the depressor mandibular or reptiles, but is innervated by the man- 

 dibular nerve and not by the facialis. The digastric is not present as such. 



Fig. 2. — Oniithorhynchus. Deep muscles. (Muscles mainly after Schulman.) 

 The anterior end of the temporalis {Tern.) is attached to a strong ligament 

 running from the postorbital region of the skull to the coronoid process of the 

 mandible. Beneath the zygomatic branch of the masseter the pterygoideus 

 externus (Pt.ext.) is seen. 



Fig. 3. — Oiflithorhynehus. 



Mandible seen from below, showing the attachments of the jaw muscles. 



Fig. 4. — The same seen from above. 



Figs. 5-7. — Echidna. (Muscles mainly after Schulman.) 



In correlation with the edentulous condition and great reduction of the jaws 

 the temporalis (Tern.) and masseter (Mas.) muscles are rather feeble. The 

 arrangement of the muscles, however, is fundamentally the same as in Orui- 

 thorhynchus. The posterior belly of the digastric of mammals is probably 

 represented by the anterior part of the stylohyoideus (St.), innervated by VII, 

 while the anterior belly is probably represented by the detrahens mandibular 

 anterior (I).m.a,), which is innervated by V. and is related to the mylohyoid 

 (Parsons). 



