528 CETACEA. 



elongated cliaracter of the sympliysis wliich, in adult females, forms nearly two- 

 thirds of the lower jaw, whereas in males it constitutes only a little more than one- 

 half of the total length of the mandihle. The rami hecome united at the symphysis in 

 utero.. Only one tooth in the young cannot be said to be symphysial, whereas in adults 

 with all their teeth intact the last tooth lies in debatable ground. In the young, 

 the symphysial portion is slightly curved upwards and forwards as far as the twenty- 

 third tooth from the first, and then forwards and downwards, there being a faint 

 upward curve at the base of the fourth tooth. The symphysial, or internal alveolar 

 plate is but slightly curved in the young skull, thus agreeing with the nearly 

 straight maxilla ; but, as age advances and the curves become more intensely marked, 

 it also partakes in the general curvature, which is adapted to the opposed curves of 

 the maxilla. This curvature is a means, in addition to the interlocking of the teeth, 

 by which the upper and lower halves of this long snout are held firmly in position 

 when the mouth is closed. 



The symphysial dental portion is strongly laterally compressed, the two 

 alveolar lines being in close apposition. The external surface is slightly concave, 

 and the middle of the symphysis is deeper than at the beginning of the ramus 

 and much more so than at its own terminal upturned distal third. The coronoid is 

 moderately high and arched, the depth of the jaw through it being less than one- 

 half of the distance between the condyle and the posterior end of the symphysis. 

 It is concave at its base externally and flat internally, and its posterior margin 

 slopes down to the condyle, which is vertically oval and directed backwards. 

 The under surface of the coronoid portion of the ramus is broad, by the inward 

 extension of the inner margin as a thin slightly outwardly curved plate, producing 

 a concavity which is continuous with the fossa along the side of the low^er border 

 of the symphysis. A short sharp longitudinal ridge for the attachment of the 

 external pterygoid muscle occurs in the concavity of the ramus before the condyle. 

 The opening to the dental foramen, on the inner side of the jaw, is large and pro- 

 longed backwards into the lower angle of the condyle, from the outer surface of 

 which it is only separated by the thickness of the condylar plate itself. In adult 

 skulls, this becomes partially absorbed, so that the two communicate. The dental 

 canal is very wide and infundibuliform. 



The lower jaw partakes of the general asymmetry of the skull, the symphysial 

 portion being twisted to the left side from its base forwards, and the free portion of 

 the rami diif ering in their curves, the left being more externally curved than the 



right. 



Hyoid (PI. XL, fig. 20). — The basihyal {bh) is a somewhat transversely cres- 

 centic flattened bone with a convex anterior and a concave posterior border, and 

 slightly concave on its upper and under surfaces, the two extremities of the crescent 

 being abruptly truncated. It is not anchylosed to the thyrohyals {th) wliich are 

 slightly curved, dilated at their basihyal ends, and contracted externally to that, 

 with a prominent nodosity in adult life on their anterior surfaces defining the 

 contraction. The ceratohyal is a long cartilaginous rod, nearly as long as the 



