1893.] cetacea:?^ genus mesoplodojS". 233 



groove in the three specimens of the younger group are in their 

 greater extent fonued by the spout-shaped vomer, by a diverticulum 

 of the maxillary bone (which appears superiorly for a short distance 

 opposite the premaxillary foramina between the vomer and the 

 premaxillary laminae), and by the premaxillaries. As the vomer 

 terminates at 3 inches from the tip of the rostrum, the anterior 

 portion of the spout is entirely formed by the premaxillaries. The 

 empty vomerine trough (in the macerated skeleton) shows no signs 

 of the cartilage, which occupied it in the living state, having been 

 attached at any point to any of the bones, except the anterior 

 termination of the ethmoid, which is rough as is usual. 



In the previous part of this paper I have shown by sections 

 what takes place in group 2 ; how, probably by the vigorous growth 

 of the maxillary and premaxillary boues surrounding the vomer, a 

 slight upgrowth, as a ridge-like elevation, appears in the bottom of 

 the vomerine troue;h, and a thickeniuff of the sides of the interior 

 walls of the premaxillaries takes place, which gradually increases 

 and eventually fills up the mesorostral groove. Where the gibbo- 

 sities occur, over the regions where the vomer does not protrude on 

 the palatal surface, this gro\\th has more space and better resists 

 the pressure, while in those parts where the premaxillaries approach 

 closer together, the increasing vomerine growth indicates by its 

 varied coutortious the effect of the strains to which it has been 

 subjected. The form, therefore, that the rostrum may eventually 

 assume in the mature animal varies x^ith the difference in the 

 strains it undergoes, through the different rate of growth in the 

 surrounding bones, and in the individual's vitality, sex, and age. 

 The various sutures and hues which are seen on the surface of the 

 solidified rostrum of aged crania have already also been explained. 



In none of the three specimens of group 1 has the buttress 

 (formed by the maxillaries, palatines, and pterygoids) extended 

 sufficiently far foruard or become prominent enough to appear 

 externally to the flanges of the basirostral groove. In the oldest 

 specimen I have examined, that from Kaiapoi (I) (Plate XIII. 

 fig. 1), the buttress and the lower margin of tlie basirostral groove 

 are x'ery prominent, and resemble closely M. australis of Elower, 

 which is also an adolescent individual. On comparing the whole 

 series, it is to be seen that the younger the age, the less anteriorly 

 does the buttress extend, and the less prominent are both it and 

 the inferior flange of the basirostral groove. 



Seen from the side. 



The boundaries of the basimaxillary groove are formed by flanges 

 of the maxillary. Tlieir disposition as seen from the npperside has 

 already been described. In the young specimen (A) (fig. 1, p. 221) 

 in the Otago Museum and in ^'an Beneden's figure, the maxillaries 

 run along the side of the rostrum, in the former to within 2*5 

 inches, and in the latter to within 35 inches of the apex ; indeed, 

 iu the latter it appears to be, at 7 inches from the tip, still -i^ of an 



Piioc. ZooL. (Soc— 1803, No. XVI. 16 



