280 AN INTRODUCTION TO ZOOLOGY 



The roof of the cranial region of the skull is composed of paired 

 frontal (m) and parietal (m) bones and posteriorly by a small inter- 

 parietal (m) bone. The f rentals are large bones forming not only 

 the roof but also the sides, and right in front also the floor of the brain 

 case. In front they meet the nasals and behind the parietals. They 

 send a very thin process forwards which separates the nasal process 

 of the premaxilla from the maxilla for some distance, and again over 

 each orbit they send out a prominent crescent-shaped supra-orbital 

 process. The parietals are somewhat smaller bones meeting the 

 frontals anteriorly, the interparietal and supra-occipitals posteriorly 

 and the squamosals along the side. The median suture where 

 the nasals, frontals and parietals of the two sides meet is termed 

 the sagittal suture. The interparietal is a small oval bone bounded 

 by the parietals in front and the supra-occipitals behind. 



The side wall of the cranial region comprises a number of bones. 

 At the antero- ventral end lies the maxilla (m). This is a complex 

 bone whose main mass forms the upper jaw and bears a series of 

 six teeth ; three are termed pre-molars and three molars. The two 

 sets are, in Lepus, indistinguishable in structure and consequently 

 all may be described as molariform. They are the grinding teeth 

 and between them and the incisors there is a long gap, the diastema. 

 Teeth corresponding with the canine teeth of other mammals are 

 lacking. The antero-lateral plate of the maxilla, forming a large 

 part of the side of the nasal capsule, is cancellous. On the ventral 

 side the maxilla sends in a short stout palatine process which joins its 

 fellow in the middle line, and these, with similar ad joining processes 

 of the palatine bone, constitute the bony floor of the narial passage : 

 this floor is the hard palate and forms also part of the roof of the 

 mouth. From the side of the maxilla is given off a stout zygomatic 

 process, which also continues upwards as an orbital process forming 

 the front wall of the orbit. The remainder of the anterior orbital 

 wall is completed by a small lachrymal bone (m) which is interpolated 

 between the orbital process of the maxilla and the frontal bone and 

 is perforated by a canal for the duct of the lachrymal gland. The 

 inner wall of the orbit is completed by the orbito-sphenoid (c), 

 palatine (m), squamosal (m) and the alisphenoid (c) bones. The 

 orbito-sphenoid is a lamella of bone surrounding the orbital fora- 

 men ; it is bounded above by the frontal, in front by the maxilla, 

 below by the palatine and behind by the squamosal. The palatine 

 is an almost vertical plate of bone forming the side walls of the 

 posterior end of the narial passage. At its front end it gives off a 

 palatine process which, as we have seen, takes part in the formation 

 of the hard palate. In addition, the palatine also forms the mid 

 ventral part of the inner orbital wall where it joins the maxilla in 



