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THE AMERICAN NA TURALIST. [Vol. XXXVII. 



the zygoma and its anterior end is wedged into the posterior end 

 of the jugal. In front, the edge of the temporal vacuity is 

 formed by the jugal externally and the postf rental internally. 

 The prosquamosal bar no longer lies parallel to the jugal bar ; 

 the jugal bar no longer extends far back to articulate with the 

 prosquamosal near the suppressed qiiadrato-jugal, but the two 

 bars lie in the same plane and unite in the middle to form a true 

 mammalian zygoma. 



In Gomphognathus the mammalian palate is fully developed 

 and the teeth are tuberculated and indic ate probably an herbivo- 

 rous habit. 



The significance of the development in the American and 

 African forms. — In a previous paper the author lias shown that 

 it is possible to recognize two lines of development in the 

 temporal region of the reptiles. The forms showing the two 

 lines were described as the mastocephalous and saurocephalous 

 groups (Case '98) as follows: "From this point (the Progano- 

 saurian condition. Diopeus') onward the Reptilia are divided 

 into two groups, one with an elongate quadrate, which includes 

 all the modern and most of the extinct Reptilia, and one with a 



Permian, and in all probability losing its identity ^n the direct 

 ancestors of the mammalia." It now seems possible to trace a 



The lower jaw is the movable element in mastication, and 

 in biting it acts as a lever of the third class with the power 

 supplied by the masseter and temporal muscles applied nearer to 

 the fulcrum, represented by the quadrate, than to the weight, 

 represented by the substance being c hewed. It is evident that 

 where the jaws are used for such heavy work as chewing the 

 fulcrum region must be very strong to resist the strain imposed 



