292 Dr. A. Fleisclimann on the Relationship 



been somewhat changed by the secondary influence of the 

 musculature there inserted ; it docs not prevail, however, 

 tliroughout the whole group, and is always absent in the 

 Hystiichida3, Subungulata, Octodontidas, Lagostomidge, and 

 Leporidaj. 



This modification, however, may be referred back to con- 

 ditions within the Marsupial series, for among them many 

 forms have lost a distinct mandibular angle, such as, for 

 example, Phascolarctoa. Then the lower jaw, if looked at 

 from the side, appears as a band dilated posteriorly into a 

 triangular plate. Nevertheless the contour of the margin and 

 the pits and bony ridges occurring on the outer surface of the 

 end of the jaw betray the previous history of the part by very 

 intelligible tokens. Even in true Marsupials we find evidence 

 of the endeavour to bring the mandibular angle from the 

 inwardly directed horizontal position into a more vertical one 

 and into the same plane as the ascending branch. In Rodents 

 all desirable steps of the retroversion have been retained, in 

 the end giving origin to the great increase of the surface of 

 the posterior extremity of the mandible. 



Side by side with this we recognize a reduction of the coro- 

 noid process ; very strongly developed in the Marsupials, it 

 is retained in all the Rodents which possess an inwardly pro- 

 jecting mandibular angle, but it becomes small until it nearly 

 disappears in Rodents with a broad mandibular plate. 



As I conceive the origin of the dentition of the Rodentia to 

 have passed through stages such as the living survivors of 

 the leaping and climbing Marsupials still display in model, 

 the dentition of their ancestors must have gradually lost the 

 omnivorous character and become herbivorous ; consequently 

 the direction of movement of the lower jaw must also at the 

 same time have become modified. 



In point of fact this transformation may be still recognized 

 from the position and form of the condylus glenoidalis in the 

 lower jaw, which passes from the transverse direction general 

 in the omnivorous Marsupials into a position parallel to the 

 sagittal plane ; and, in accordance with this, the cavitas gle- 

 noidalis on the squamose part of the temporal, which in the 

 Marsupials attains no great extension, becomes gradually 

 longer so as to pass on to the jugal arch and become a long 

 groove-like excavation. 



The occurrence of the change of food may be further in- 

 ferred from the constitution of the digestive organs in the 

 Rodentia. I indicate now only the form and structure of the 

 stomach. Whilst in most Rodents this possesses a pretty 

 simple structure and form, it becomes more highly compli- 



