502 CETACEA. 



where they hend forward to meet the squamous and parietals. The precondyloid 

 foramen is large and well-developed, with the cartilaginous stylohyal rod firmly 

 applied to it sideways and hending round the posterior extremity of the tympanic, 

 from hehind forwards, and between it and the par occipital process to the periotic. 

 The facial nerve is seen coming out through a foramen formed by a notch in the 

 anterior margin of the exoccipital, immediately external to the cartilage aforemen- 

 tioned and the paroccipital process. 



The exoccipitals (PI. XL, figs. 3 and 4), in a young skull in which they are 

 quite distinct, do not form the upper margin of the foramen magnum ; hut the 

 condyles are almost entirely formed by them, only a fifth share being contributed 

 by the basi-occipital. The anterior exoccipital face is concave internal to the 

 transverse ridge and has the precondyloid notch immediately internal to the parocci- 

 pital process. The notch in the skull is converted into a foramen. The transverse 

 ridge terminates above the inner border of the paroccipital process on a line with 

 the anterior basi-occipital angle of the bone ; the whole of the paroccipital process 

 below that level is outside and deeply concave, especially at its lower margin, 

 where a distinct pit is formed. A small but deep fossa occurs external to the 

 lower end of the transverse ridge, separated by a fine ridge from the paroccipital 

 concavity, into which the posterior superior external angle of the periotic abuts. 

 The lower concavity faces the posterior surface of the periotic and pars mastoidea 

 and the hinder aspect of the tympanic cavity, being separated from these bones 

 by the stylohyal cartilage passing in behind the tympanic bone and posterior to the 

 facial nerve, which is protected anteriorly by the paroccipital process. 



In a skull measuring 12 inches long, in which the supra- occipital has not been 

 amalgamated with the exoccipitals, the latter bones are completely united to the 

 basi-occipital ; and in this skull there is an imperfection of ossification representing 

 the posterior condyloid foramina, which are l^almost always present either on one or 

 both sides associated with a thinning in this region where they do occur. 



The paroccipital process becomes much thickened with age, and in an adult 

 male its external margin below is grooved for the passage of the facial nerve, while 

 its anterior aspect is much roughened ; the concavity all but disappearing, the lower 

 pit being occasionally converted into a short canal by a broad spiculum of bone 

 arching over it. 



The internal surface of the paroccipitals in the adult skull has the transverse 

 ridges greatly developed and grooved for the lateral sinuses. Immediately external 

 to the occipital protuberance, they are indented on their under surfaces by a rather 

 deep pit, which is overhung above by their sharp edges. This pit is continuous with 

 a broad and deep furrow that lodges the posterior occipital sinus. There is no trace 

 of an internal occipital crest nor of a falx-cerebelli. 



Basi-occipital (PI. XL, fig. 5). — The supra-occipital would appear to unite 

 with the exoccipitals about the same time that the basi-occipital amalgamates with 

 the basisphenoid. In the very young skull it is perfectly flat on its under surface 

 between the weU- developed tympanic borders, with the exception of a thickened 



