IOO 



THE AMERICAN NATURALIST. [Vol. XXXVII. 



lation of the lower jaw finally comes at the base of a strong 

 buttress formed by the coossified bones of the suspensorium. 



The second of the two processes, described above, was 

 attempted in two different ways. The flattening of the quadrate 

 was common to both, but the supporting temporal arches were 

 disposed of very differently. In the American Pelycosaurians 

 the bones of the temporal arches all retained their identity and 

 remained distinct one from the other, but the posterior ends of 

 the bones followed the quadrate down in its degeneration until 

 the angle of the skull was depressed and the arches were long 

 and slender; compare figures 3 and 8. This made a very weak 

 suspensorium especially as the opisthotic, the single bone which 

 connected the quadrate directly with the skull wall was separated 

 from it by the intervention of considerable cartilage. When the 

 jaw was compressed on food in the act of biting, the strain on 

 the quadrate region would be directly upward, but instead of 

 this strain being met by bones arranged to meet it parallel to 

 their length and firmest attachment, as in the modern lizards, 

 or against bones solidly united and joined to the brain wall, as in 

 the mammals, it was directed almost at right angles against the 

 free ends of slender bones at a point fartherest removed from 

 their attachment to the skull. Such an arrangement was clearly 

 unadapted to resist the strain imposed upon it by the force of the 



above, the advance in the slendern'ess of the quadrate regie m'kept 

 pace with the advance in the development of the carnivorous 

 habits as evidenced by the increase in size of the incisor and 

 canine tusks and the serrations of the cutting edges of the 

 teeth ; the latter speaking of a growing habit of sectorial masti- 

 cation. The two processes working in opposition. 



<>J'j^ the interior end of the jngal or, more probably 

 After the union of the individual bones the twe^ Trches 



