113 -l.V.Y.^L-S A'EW YORK ACADEMY OF SVIEXCES 



also very aberrant, so that it is not strange to find different conditions in 

 the miiscuhiture. The jaw muscles of monotremes include the following: 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V^) 



Masseter Ptervgo-tympanicus 



Temporalis Depressor mandibuls anterior 



Pterygo-spinosus Detrahens mandibulas 



MUSCLES OP THE DIGASTRIC GROUP 

 (INNERVATED BY VII) 



These are not present. 



Monotremes have the following peculiar conditions of the jaw muscles : 

 They have no true digastric. The masseter and temporal muscles are 

 massed together. They have no pterygoideus internus. Presence of the 

 pterygo-spinosus and pterygo-tympanicus. Presence of the detrahens 

 maudibulge instead of the digastric. Extensive development of the mylo- 

 hyoid group of muscles. 



Absence of the digastric. — The digastric is fairly constant in the mam- 

 mals except monotremes, edentates and some Cetacea. Perhaps it is ab- 

 sent in the monotremes because they separated from the mammal stem 

 at a very early period and thus have missed this arrangement. Ornitho- 

 rhynchus with its fairly large jaws needs a depressor, and this function is 

 filled by the detrahens muscle and by the depressor mandibuls anterior 

 of the hyoid group. Echidna is so degenerate in the mandibular region 

 that there is very little need for a specialized depressor. Other mammals 

 in which this muscle is missing have degenerate jaws. Chaine mentions 

 Tatusia of the edentates and Delphinus of the Cetacea, where the true 

 digastric has been lost. 



Imperfect separation of the masseter and temporal. — -The condition of 

 the masseter-temporal mass in the monotremes is very suggestive of the 

 capiti-mandibularis in the reptiles, as it is not differentiated as much as 

 in other mammals. The mass has several heads, but the separation is not 

 so complete. The peculiar shape of the skulls in the monotreme may be 

 responsible for this condition. There is very little room for muscles in 

 Echidna, as the mass is completely covered by bone. Schulman (1906) 

 gives a division of the muscles showing their complexity and tendency to 

 break up into small slips. 



Absence of the internal pterygoid. — The absence of the pterygoideus 

 internus is striking, as it is one of the constant muscles of the mam- 

 malian skull. Some writers gave the monotremes an internal pterygoid, 



