IVIE. W. H. PLOWEE ON THE OSTEOLOGY OF THE SPERM-WHALE. 329 



Inia, Pontoporia, Beluga, and Monodon* all are separate, as in the Balcenojiteridoe. In 

 the other Delphinoids the atlas, axis, and generally one or more of the other ve];tebrEe 

 are confluent ; if any are free, it is at the hinder end of the series. 



The form of the atlas of the Cachalot is very characteristic. The great vertical depth 

 of the obliquely truncated transverse processes, and the comparative straightness of the 

 upper and lower border, especially the former, give it, when seen either from before 

 or behind, a transversely extended quadrangular figure, quite unlike that of any other 

 Cetacean atlas. It of course partakes of the regional character of great antero-posterior 

 compression, though not to the same extent as the succeeding vertebrae. The whole 

 bone, with all its inequalities, will lie between two planes 8" apart ; and nowhere does 

 the actual thickness exceed 6". The anterior surface is hollowed out to a depth of 4^" 

 for the reception of the condyles of the occiput. The posterior surface is remarkable 

 for its flatness. 



Fig. 2. 



Anterior sm-face of atlasf. 



The form of the neural canal, where it pierces the atlas, is very nearly that of an 

 equilateral triangle, with one of the angles directed downwards. The upper side is 

 almost straight, the outer angles rounded : the lateral sides converge rather rapidly to 

 a point rather below the middle, where their posterior margins form an angular pro- 

 jection, causing a constriction of the opening. Below this the sides approach more 

 gradually towards the inferior angle, which is truncated at the apex. If the anterior 

 margin of the aperture alone could be seen, it would appear more perfectly triangular, 

 with straight sides. Seen from behind, the opening appears divided by the above- 

 mentioned projection on the posterior margin into two parts, an upper transversely 

 elongated oval portion, and an inferior narrow vertically elongated part. The former 

 alone corresponds to the neural canal of the succeeding vertebrae ; the latter fits over the 

 rough surface of the axis to which the odontoid ligaments are attached, and affords a 

 passage for them. 



* In two skeletons of male Narwhals in the Mus. Roy. Coll. Surg, the bodies of the 2nd and 3rd cervical 

 vertebrae are firmly united. 



t The woodcuts of the vertebrae (as the figures in Plates LVIII. and LIX.) are all reduced to jlj the size of 

 nature. The anterior surfaces are represented in every case. 



