26 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 57. 



in its boundary and the squamosal furnishes the principal part of 

 the boundary behind, but with the great changes that have occurred 

 in this latter bone in the transition from reptile to mammal it is 

 not surprising that the vacuity should have been shifted forward 

 slightly and the squamosal entirely withdrawn from its boundary. 

 In Tupaia the anterior end of the zygomatic process of the squamosal 

 lies just behind it, and the change from the relations exhibited in 

 ProcoIopJion to those seen in Tupaia would be very slight indeed. 



Upon the whole, therefore, the presumption that the so-called 

 malar foramen of Tupaia, of certain of the })rimates, and possibly 

 of the fruit-eating bats, is the remains of the lateral temporal vacuity 

 of the reptilian skull, finds a fair and reasonable measure of support. 



The homology of the vacuity lying just above the temporo- 

 mandibular articulation described in the Monotremes is not difficult 

 to discover when we compare it with such a type as Splienodon. In 

 this latter form the large opening from the temporal area on each 

 side of the braincase, which is directed backwards (the supraoccipital 

 vacuity) has for its boundaries above the squamosals and parietals, 

 and belov/ chiefly the exoccipitals. As will have been noticed in 

 the description of this aperture in the duckbill's skull this is almost 

 the exact boundary there seen. The only difference is that in 

 Splienodon, because of the small brain case the parietal enters into 

 the bounding arch above, while in the Monotremes, owing to the 

 enlargement of the cranial cavity, the parietal has been excluded, 

 and the outer part only of the vacuity in the reptile skull is repre- 

 sented in the mammal. This homology is so clear and unmistakable 

 ihat there is no room for doubt as to its correctness. If this conclu- 

 sion is correct then the similar opening seen in certain Rodents, 

 notably in Lophiomys and in the Marsupials is the inconsiderable 

 remnant of the same supraoccipital vacuity of the reptilian skull. 



Fate of the Reptilian quadrate in tJie Mammalia. — The determina- 

 tion of the homology of the premallear element or elements described 

 in the foregoing pages involves a discussion of some of the most 

 important problems connected with the morphology of the mamma- 

 lian skull. The great question, " What has become of the quadrate ? " 

 in the evolution from the reptilian to the mammalian condition has 

 fretted the minds of philosophers and baffled the best brains of 

 morphologists for the last 50 years without any generally accepted 

 anil satisfactory answer. Without attempting to go into the exten- 

 sive literature on the subject and follow out in detail the various 

 theories that have been advanced, it must suffice here to say that the 

 subject has finally settled down to two rival theories— namely : 

 (1) The reptilian quadrate has been detached from its original 

 position as a suspensorium of the lower jaw, has been much reduced 

 in size, and has become the middle element of the ossicular chain of 



