On the Geological Distribution of the Rhabdophora. 45 



others are subcuneiform. The inferior canines are consider- 

 ably larger than the incisors. The latter are regular and do 

 not overlap each other ; the second and third inferior pre- 

 molars have well-developed basal lobes anteriorly and poste- 

 riorly. The heel of the sectorial is well developed. The 

 tubercular is very small. 



The form of the skull is short and wide ; the zygomata are 

 much expanded ; and the profile is very convex. The muzzle 

 is short, and the orbits are rather large. The interorbital 

 region is wide and convex ; and the postorbital processes are 

 robust, acuminate, and directed downwards. The infraorbital 

 foramen is very large. The apices of the premaxillary bones 

 are elongate, but do not reach the frontals. The nasals are 

 rounded posteriorly. The sagittal crest is prominent, and the 

 inion elevated. The posttympanic process is short ; and the 

 paroccipital is short and is directed backwards. The cranium 

 is constricted behind the orbits. The mandibular ramus is 

 low posteriorly ; and the anterior inferior flange is well deve- 

 loped, but not large. 



Measurements. 



metre. 



Length of skull on base -140 



Width of skull, measured below "Ill 



Length of palate -060 



Width of palate between posterior angles of sectorials "062 



Width of palate between canines "026 



Length of skull to front of orbits (axial) '050 



Vertical diameter of orbit -031 



Interorbital width ( least) -045 



Elevation of inion from foramen '082 



Length of inferior molar series "050 



Length of inferior sectorial "018 



Length of base of inferior first premolar # 055 



Depth of ramus at sectorial "016 



Depth of ramus at first premolar "021 



Depth of ramus at flange '026 



From the Truckee beds of John Day River, Oregon. 

 [To be continued.] 



V. — On the Geological Distribution of the Rhabdophora. 

 By Charles Lapworth, F.G.S. &c 



Part II. Data. 

 [Continued from vol. iv. p. 431.] 



Silurian System (Upper Silurian of Murchison). 

 Of the manv suggested modifications of Murchison's latest 



