SKULL — MAMMALS 



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a splint between these {e.g., rats). Other genera have a large zygomatic, forming 

 the lower border of the orbit and even extending to the lacrimal, and sometimes 

 back so that it forms part of the mandibular fossa. 



The supraoccipital extends to the roof, and in many Simplicidentata has a 

 lateral process on either side, which may He on the exoccipital, and sometimes 

 connects with the squamosal. An interparietal is common. The nasals, large 

 and sometimes fused, always over-arch the terminal nares. Each nasal joins 

 the premaxilla, which, because of the large incisors, is large and extends back to 

 the frontal, and has a large diastema on its alveolar margin. A peculiarity of 

 many genera is that a part of the masseter muscle passes through the infraorbital 

 foramen, which is correspondingly enlarged and may unite with the orbit 

 (fig. 221, B). The perforate lacrimal lies within the orbit. The mandibular 

 fossa, which lacks a postglenoid process, permits a fore and aft motion of the 

 lower jaw, but in some the sides of the fossa are elevated, otherwise, as in beaver, 



Fig. 221. — .4, Skull of Hydrochoerus capybara (Zittel, '93); B, oi Ccelogenys paca 

 (Pouchet et Beauregard, '89). a, alisphenoid; an, angle of lower jaw; b, tympanic 

 bulla; c, condyle; d, dentale; /, frontal; /, lacrimal; m, maxilla; w, nasal; 0, occipital; 

 p, parietal; pe, petrosal; pm, prema.xilla; pp, paroccipital process; sq, squamosal; 

 2, zygomatic. 



lateral motion is possible. Some have a separate optic foramen, in others the 

 nerve passes through the orbital fissure. Foramina ovale and rotundum are 

 present except in porcupines and Duplicidentata. 



The incisive foramen (sometimes slit-like) is behind the incisors. The hard 

 palate is narrow, and frequently has a deep incision behind, the palatal processes 

 being reduced. The presphenoid is prominent. The petrosal is loosely con- 

 nected with the other bones, except that the tympanic fuses with it, but not 

 the squamosal. The mastoid, visible between ex- and supraoccipital and 

 squamosal, is often swollen to a vesicle connected with the tympanic cavity, 

 and may reach parietal and interparietal. Posttympanic and paroccipital 

 processes vary. The mandible varies in size and height of its ramus and its 

 coronoid and condyloid processes, and there is always an angular process which 

 encloses a groove on the medial side for the insertion of the pterygoid muscle. 



TiLLODONTiA are rodent-Uke in dentition, have low elongate skulls with 

 sagittal crest and low brain capacity. They have a complete zygomatic arch, 

 the zygomatic bone not reaching the mandibular fossa. Orbits and temporal 

 fossa are widely connected; the lacrimal foramen is in the large facial part of the 



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