Mammalia. Marsiiplalia. 53 



formed by the Squamosal ; but presents various forms in direct cor- 

 relation with tlie structure of the teeth, and the movement required 

 of the jaw. The angle of the Jaw is as if it were lent inward* in the 

 form of a process. On looking directly upon the lower margin of the 

 jaw, in place of the margin of a vertical plate of bone there is seen 

 a more or less triangular surface, extended between the external 

 ridge and the internal process, or inflected angle (esp. in "Wombat).f 

 In all those Marsupialia which have few or very small incisors the 

 horizontal rami converge towards a point, the symphysis being more 

 or less complete. The Nasal cavity communicates with the mouth by 

 means of various large vacuities in the palatal processes. Two pos- 

 terior Palatal Foramina are generally present, of variable size and 

 position, especially noticeable in Perameles lagotis, in which the bony 

 roof of the mouth is perforated by a wide oval space, exposing 

 to view the Vomer and the convolutions of the inferior Spongy 

 bones in the nasal cavity. The Pterygoids long maintain their 

 individuality, and are always small and lamelliform. The Entocarotid 

 canals pierce the Basisphenoid, as in Birds. The Tentoritim is some- 

 times partially, but never completely ossified in any of the Marsu- 

 pialia. There is no ossification of the Falx, as found in Ornitho- 

 rhynchus [cf. Monotremata). There is no distinct pituitary fossa^ 

 nor Clinoid processes. The Optic foramen is confluent with the 

 sphenoidal fissure. 



The Vertebral Column. The inferior arch of the Atlas frequently 

 remains permanently open in the middle line below {e.g. Phascolomys, 

 Phascolarctos, Phalangista, and Macropus). This opening is generally 

 occupied, in carnivorous Marsupialia and in Thylacinus, by a distinct, 

 and separate ossification. In all Marsupialia the spine of the Dentata 

 is well developed, both in the vertical and longitudinal directions. 

 The transverse processes of the Cervical vertebra? are generally more 

 or less expanded nearly in the axis of the spine, so that the pos- 

 terior part of one transverse process overlaps the anterior part of the 

 succeeding. The spine of the first Porsal is generally considerably 

 longer than that of the last cervical, and all the spines generally 

 slope towards the centre of motion. The Eils consist of bony pleur- 

 apophyses, and gristly ha3mapophyses, which acquire bone earth 



* Except Tarsipes. 



t The outer surface of the ascending ra7nus is imperforate in the Dasyuridae, 

 Didclpliyda?, Peramelidao, and Phalangistidse ; but in the Phascolomyd(e and 

 Macropidfc it is Aixecily perforated by a round aperture immediately posterior, 

 or oppoi^ite to the commencement of the dental canal. 



