SKELETON OF PHOEOEHACOS INFLATUS. 57 



depressed area lying between them and the foramen magnum. This valley-like 

 depression is closed above by the meeting of the supraforaminal ridges (sf.r.), but, 

 opens ventrally on either side of the occipital condyle. The inferior angles of the 

 paroccipital processes are united to the mammillary tuberosities (m.t.) of the basi- 

 temporal platform by prominent buttresses of bone, behind and internal to which the 

 vagus foramina open. 



The foramen magnum (oc.f.) is relatively very small; it is oval in outline, the long 

 axis being vertical. The plane of the opening is inclined backward, making an angle 

 of rather more than 45° with the long axis of the skull. The transverse diameter of 

 the opening is 9 mm., the vertical 16 mm. 



The slightly pedunculate occipital condyle (oc.c.) is oval, or rather reniform, its 

 upper border being nearly straight, with a vertical groove running down from its 

 middle point nearly to the centre of the condyle. There is a small but deep pre- 

 condylar fossa (pc.f., Plate XV. fig. 1). 



The base of the Skull (Plate XV. fig. 1). — The basitemporal platform (b.t.) is a 

 triangular area which is very slightly elevated, and below the level of which the 

 occipital condyle projects. It is concave from side to side, and the postero-lateral 

 angles are produced downward into the very prominent mammillary tuberosities 

 (m.t., fig. 1), which, as mentioned above, are united to the inferior angles of the 

 paroccipitals by buttresses of bone. The posterior margin of the platform is defined 

 by a shallow groove separating it from the occipital region. The lateral borders are 

 thickened ridges, forming anterior prolongations from the mammillary tuberosities 

 (m.t.), and converging so that they appear to have met in front in a rounded median 

 angle. This, no doubt, lies immediately below the common opening of the eustachian 

 tubes, which are immediately dorsal to the thickened ridges just described, and open 

 posteriorly by wide apertures into the tympanic cavity. In front of the basitemporal 

 platform the narrow rostrum (r.) is seen for a short distance, but it is here badly 

 preserved, and anteriorly is completely invested by the bones of the palate. 



On the sides of the basitemporal platform, rather high up and behind its anterior 

 angle, are a pair of basipterygoid facets, with which the pterygoids articulate by means 

 of stout processes borne on their inner surfaces rather behind their middle point (b.pi.). 

 Below and behind the articular facets for the pterygoids the basitemporal forms a thin 

 plate of bone (pretemporal wing), which looks outward and downward ; the lower edge 

 of this forms the floor of the eustachian canal, while its upper portion is the outer 

 wall of the large pretympanic recess, which opens widely into the tympanic cavity 

 immediately above the posterior aperture of the eustachian tube. 



The dele of the Stall (Plate XIV.).— The tympanic cavity (ty.) is relatively small ; 

 posteriorly it is bounded by the concave anterior face of the paroccipital process ; the 

 inferior portion of its inner wall is formed by the buttress of bone which joins the 

 paroccipital to the mammillary tuberosity (m.t.). Its outer edge is formed by a 

 downwardly-projecting flange of the squamosal, the anterior edge of which runs down 



¥2 



