214 VERTEBRATE SKELETON 



The lacrimal bone, perforated by one or more foramina, is variable (small in 

 Diocotylcs, facial and orbital parts about equal in Ruminants, facial part large 

 in Hippopotamus). The maxilla is large to accommodate the large teeth, and in 

 antelopes it is excavate in places for dermal glands. The reduced sphenoid 

 has optic, oval and round foramina, but an ahsphenoidal canal is lacking and 

 there is an ectopterygoid fossa except in a few Suids. The squamosal is more of 

 an element in the cranial floor than in most mammals. The hard palate is long, 

 the petrosal small and, as a rule, loosely connected with the rest of the cranium. 

 The tympanic forms a long bulla-like structure, excavate only in Pecora. The 

 mandibular fossa is wholly squamosal and is bounded by a postglenoid ridge or 

 process; its axis is transverse, but permits lateral motion in Ruminants. The 

 lower jaw is all but universally slender, has a high ramus and the halves are 

 united by cartilage at the symphysis. 



In non-Ruminants the tympanic fuses with the squamosal, forming a long 

 osseous meatus. Ruminants have a complete postorbital bar. Tylopoda lack 

 horns or antlers on the frontals, these being well developed in at least the males 

 of most Pecora. Both horns and antlers are parts of the skeleton. True horns 

 consist of permanent processes on the frontals (horn cores) covered with horn 

 derived from the skin (mostly epidermal). Antlers are formed annually in 

 the male by outgrowths from the frontal bones (p. 195), these being covered at 

 first by skin and hair ('velvet')- This is soon rubbed off. At yearly intervals 

 the bone of the antler is shed and later replaced as described earlier. Of the 

 true (Cavicorn) horns, the pronghorn of western America differs from all others 

 in the shedding of both horn and core. Giraffes have a tendency towards the 

 formation of a third horn on the nasal bones, this like the other two horns of 

 these animals being covered by skin. 



Perissodactyla. — In Tapirus and Rhinoceros, as in most of the ancestral 

 forms, orbit and temporal fossa are continuous, but in recent Equidae the orbit is 

 closed behind by a strong process from the frontal to the zygomatic process of 

 the squamosal. Tapirs and horses have a moderate sagittal crest. The 

 interparietal fuses with the parietals; the nasals, expanded behind, are separate 

 except in Rhinoceros where they support the horn, (the second horn of Diceros 

 is on the frontals), the horns being solely dermal. In tapirs the nasals are short, 

 the nares at about the level of the orbits. The ascending process of the pre- 

 maxilla extends to the nasals in the Equidae, bounding the naris laterally; it is 

 shorter in other groups, the maxilla sometimes bounding the naris. The 

 premaxillse are fused in tapirs, and in all Perissodactyls the palatal processes are 

 small. The maxilla; often extend back beneath the zygoma to accommodate the 

 large molars, and dorsally they enter the floor, but not the margin of the orbit. 

 The facial part of the lacrimal is large in tapirs, the foramen (divided) being 

 in the orbital part. 



The alisphenoid usually has an ahsphenoidal canal for the maxillary artery, 

 but there is no foramen ovale. The hard palate is largely maxillary, the pala- 

 tines being small. The mandibular fossa (wholly squamosal) is shallow and 

 short, with a postglenoid process. Tympanic and petrosal are ankylosed, except 

 in tapirs, and are separated from the occipital and sphenoid by a wide cleft which 



