466 ANATOMY OF VERTEBRATES. 



up a process to the postfrontal, which it rarely reaches : it ex- 

 tends backward to the glenoid cavity, and forms the under part of 

 the zygoma : the upper part is due to the squamosal, 27. The ex- 

 ternal nostril is terminal, vertical, and formed by the nasals and 

 premaxillaries : the maxillaries are perforated by a moderately 

 large antorbital foramen far in advance of that cavity : the lateral 

 series of molar alveoli slightly diverge anteriorly — a disposition 

 which Cuvier regarded as peculiar to the Hippopotamus among 

 Mammals. The bony palate is deeply notched anteriorly between 

 the premaxillaries : there are two pairs of ' foramina incisiva.' The 

 ascending ramus of the mandible, i, has a j)Osterior convexly- 

 curved outline descending to an antroverted angular process ; 

 the horizontal rami, divided by a deep notch from the angle, run 

 forward almost parallel with each other, and expand at the sym- 

 physis, along whose upper and anterior broad truncated border 

 the incisor sockets, four in existing, six in some extinct. Hippo- 

 potami, form a straight transverse line, between the tusks, c. 



In a very young Hippopotamus may be observed the following 

 evidences of cranial structure. The basioccipital has partially 

 coalesced with the basisphenoid, but not with the exoccipitals ; it 

 forms no part of the occipital condyles, and developes no processes 

 from its under surface : its lateral synchondrosal surfaces are 

 divided into two facets, one for the part of the exoccipital behind 

 the precondyloid foramen, the other for the smaller part in front. 

 These parts of the exoccipital have not coalesced on the inner 

 side of that foramen, which is single : the exoccipital developes, 

 besides the condyloid process, the i:)aroccipital and a broad process 

 to join the mastoid. The superoccipital is a thick, rhomboid, 

 vertical plate. The alisphenoids have coalesced with the basi- 

 sphenoid : they are short, and are grooved behind by the boundary 

 which they contribute to the foramen common to the foramen 

 ovale and the basicranial foramen lacerum, and more deeply in 

 front by the part they contribute to the foramen common to the 

 foramen rotundum and foramen lacerum anterius : they develope 

 long pterygoid processes, which are imperforate, and articulate 

 along their inner sides with the entopterygoids. The presphenoid 

 has coalesced with the orbitosphenoids and with the rudimental 

 prefrontals, which are connate, compressed, and form the median 

 septum of the great anterior outlet of the cranial cavity. The 

 vomer is a long, slender, pointed bone, deeply grooved above. 

 The parietals articulate with the alisphenoids, orbitosphenoids, 

 squamosals, mastoids, frontals, superoccipital, and each other. 

 The under part of the frontal is divided into a cranial, orbital, and 



