ExPLA.\ATio,\ OF Plate IX 



Jaw muscles of mouotremes : OrnitliorJiijnchus, Echidiiii. After Scluilman 



The jaw muscles of monotremes as well as their skulls exhibit peculiai" 

 specializations along with certain very ancient characters not found in otlier 

 mammals. 



Fig. 1. — OrnUhorhijiiehufi. Superficuil muscles. (Muscles mainly after Scliul- 

 mau.) 

 The capiti-mandihularis of reptiles is now represented by tlie raasseter 

 (Mas.), temporalis {Ton.}, and pterygoideus externus. The masseter is sub- 

 divided into several muscles of which the detrahens mandibulic (Det.in.) 

 simulates the depressor mandil>ul;e or rei)tiles. but is innervated by the man- 

 dibular nei've and not by the facialis. The digastric is not present as sucli. 



Fig. 2. — Ornilhorhynchus. Deep muscles. (Muscles mainly after Scliulman.) 

 The anterior end of the temporalis (Ton.) is attached to a strong ligament 

 running from the iwstorbital 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. o. — Ornithorhijnch us. 



Mandible seen from Ijelow. 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 mas.seter {Mas.) muscles are rather feeble. The 

 arrangement of the mu.scles, however, is fundamentally the same as in Orni- 

 fhorht/nchus. The posterior belly of the digastric of mammals is probably 

 i-epresented by the anterior part of the stylohyoideus (<S7.). innervated by VII, 

 while the anterior belly 4s probably reiiresented by the detrahens mandibula> 

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

 (Parsons). 



