OSTEOLOGY OF CARNIVOROUS DINOSAURS. 145 



1 . Known change of slender, openly constructed skull of earlier forms to shortened : 

 reduced fenestration with correspondingly expanded bony structures, which reaches 

 its culmination in the Upper Cretaceous Tyrannosaurus. 



2. Numerical reduction of the premaxillo-maxillary dental series. 



3. A reduction in size of the prefrontals. 

 To these may now be added : 



4. A change from the lengthened movably articulated quadrate in the Triassic 

 and Jurassic Therdpods to the much shortened and fixed quadrate of the Upper 

 Cretaceous forms. 



The specialization of the fore limb and foot in the carnivorous dinosauria during 

 successive geological periods now appears to be (1) a relative diminution in the size 

 of the entire fore limb; (2) a reduction of the number of digits; (3) the elongation of 

 the penultimate phalanges; (4) lengthening of the scapula. 



Beginning with the Triassic Theropods all are found to possess the full comple- 

 ment of five digits, though the fifth, as in AncMsaurus and the European Plateosaurus 

 is usually reduced. In the Jurassic we find in Ornitholestes that the fifth digit has 

 entirely disappeared and the fourth is represented by a vestigal metacarpal. An- 

 trodemus has gone still farther, and apparently the fourth has entirely disappeared, 

 with a considerable reduction of the third, and approximately the same condition 

 obtains in the hand of Compsognaihus from the Upper Jurassic of Bavaria. Cera- 

 tosaurus is exceptional in having four digits, with both the first and fourth digits 

 undergoing reduction. In the Upper Cretaceous we have in Gorgosaurus from the 

 Belly River formation a still further reduction, there being only two functional 

 digits, the third being represented by the vestigal metacarpal. The hand of Tyran- 

 nosaurus from the Lance is not yet known, but it appears quite probable that it will 

 be found to be functionally didactyle. 



In the hind feet we find that the Triassic carnivores all have five digits, the first 

 and fifth undergoing reduction. So far as known at this time all of the Jurassic forms 

 with the exception of Compsognaihus have lost the fifth digit, with a further reduction 

 of the hallux. While in the Upper Cretaceous genera the first digit has become 

 relatively smaller and in Ornithomimus to have disappeared altogether, there is still a 

 vestigal fifth present, as there is in Gorgosaurus. In Tyrannosaurus the fifth appears 

 to have disappeared, though the first is still present and apparently not reduced much 

 beyond the condition found in the Morrison Theropods. 



The presence of the vestigal fifth in the Upper Cretaceous Theropods suggests 

 that perhaps it is also present in such Morrison forms as Antrodemus and Ceratosaurus 

 but has not yet been discovered. It is also to be noted that the rather loosely 

 articulated metatarsals of AncMsaurus and Plateosaurus of the Triassic progressively 

 become more and more compactly united, thus exhibiting a more powerful and less 

 mobile arrangement of the metatarsals, which in this respect attains the highest 

 degree of specialization in the Upper Cretaceous genera. 



The astragalus also exhibits a progressive development of the ascending process, 

 as shown by its absence, or, at the most, an incipient stage in the Triassic Theropoda, 

 the intermediate stage in the Morrison forms, and its maximum development in the 



