408 PKOP. T. JEFFERY PAEKEK ON THE CRANIAL OSTEOLOGY, 



Absolute measurements, as defined on pp. 406-407, of the various species examined, 

 are given in Table A. 



But, as an aid to the determination and definition of the genera, the proportions of 

 the skull are more important than absolute measurements,' a fact recognized by Hutton, 

 who gives (9, p. 107) the proportion between length and breadth, and between breadth 

 and height, in his eight genera. I have come to the conclusion, however, that a more 

 convenient method is to take as a standard the length of the basis cranii as defined 

 above (p. 406, § 3), and to express various other important dimensions as percentages. 

 In this way a number of indices are obtained, many of which are of great importance 

 in the definition of the genera : they are given in Table B. 



Table C gives what may be called the temporal index ; i. e. the width of the 

 cranium at the temporal fossa as compared with the distance between the right and 

 left temporal ridges. 



6. SUMMAET OP THE CeANIAL CHARACTERS OP THE RaTIT^. 



Stkuthio. 



Occipital plane inclined backwards ; occipital condyle sessile; a broad occipital crest. 



Length of cranial roof nearly three ti mes that of basis cranii. 



Width at paroccipital processes about one and a half times length of basis cranii. 



Width at squamosals about double length of basis cranii. 



Height of cranium nearly double length of basis cranii. 



Temporal fosste continued mesiad on to parietal region ; distances between temporal 

 ridges about one third less than width of cranium at temporal fossae. 



Zygomatic process outstanding ; no squamosal prominence. 



Width of orbit about equal to breadth of cranium at paroccipital processes, or one 

 and a half times length of basis cranii ; interorbital septum present ; a projecting 

 supraorbital ledge notched in front and produced behind into a short blunt, post- 

 orbital process. 



Lacrymal produced into a broad, backwardly-directed orbital process, and a strong 

 descending process passing mesiad of the lacrymal duct. 



Nasal has a well-developed maxillaiy process, and is separated posteriorly from its 

 fellow by an interval in which the mesethmoid appears. 



Ossified portion of antorbital does not extend outwards to lacrymal. 



PremaxiUa weak ; body flat, having no prenarial septum ; no palatine processes ; 

 width of body about equal to length of basis cranii. 



Maxilla narrow; extends forwards to body of premaxilla; maxillo-palatine a 

 hatchet-shaped process given off fi-om its mesial border; antrum small. 



