164 MAMMALIAN ANATOMY 



exoccipitals ; the basilar plate is often called the basioccipital. It 

 presents for study two surfaces, three borders, and three angles : 



The posterior or external surface is triangular, with the apex 

 superior and the base inferior. It is separated into (1) a' narrow 

 anterior part and (2) a large posterior part by the superior curved, 

 line which assists in producing the lambdoidal l ridge of the skull. 



(1) The anterior part might be termed the expanded portion of 

 the upper part of each lateral border (Fig. 109). It is not seen when 

 the bone is viewed from behind, and it is the only surface of the bone 

 which is visible when the skull is viewed from above. It is divided 

 into right and left halves by the median enlargement of the superior 

 curved line, known as the external occipital protuberance. This 

 protuberance is triangular, the anterior angle being prolonged forward 

 to join the longitudinal crest of the interparietal. The surface on 

 each side of the protuberance is long and narrow, usually three times 

 as long from above downward as it is wide from before backward. 

 The posterior margin is formed by the curved line, and is elevated 

 and regularly arcuate. The anterior margin is the upper part of 

 the lateral border of the occipital plate, and is almost straight ; it is 

 bevelled to a varying degree, at the expense of the surface, for 

 articulation with the interparietal and the parietal. It begins above 

 at the front of the external occipital protuberance, and passes down- 

 ward and slightly forward and ends below the middle of the distance 

 measured from the protuberance above to the inferior angle below. Its 

 termination forms a distinct angle with a short, irregular, transverse 

 margin. The surface varies in length from before backward, accord- 

 ing to the amount of bevelling of the anterior margin. It is convex 

 from above downward and concave from before backward, the amount 

 of concavity depending on the prominence of the lambdoidal crest. 

 It is perforated by small, inconstant foramina leading to the diploe. 

 The rough surface below the transverse lower margin is called the 

 lower part of the lateral border of the occipital plate, and articulates 

 with the temporal bone. 



(2) The larger, posterior part of the external surface of the 

 occipital plate (Fig. 110) is nearly vertical. It is bounded on each 

 side, above, by the curved line ; below, by the lateral border. The 

 general direction of the lateral boundary is downward and outward. 



1 From lambda, the Greek letter L, and eides, like. 



