230 ME. H. o. FOEBES OS THE [Feb. 28, 



Mesoplodon Jiaasti, Flower, Tr. Z. S. x. p. 419 (1878). 



Oulodon f/rayi, Haast, Van Beneden & Grervais, Osteog. des Get. 

 p. 516, pi. Ixii. 



In examining the list of specimens of this species which I have 

 enumerated above, they fall into three groups : — (1) The young 

 forms in wliich the mesorostral groove is empty (A, Ac, B) ; (2) 

 those in which the groove is solidly filled up with porcellaneous 

 ossification (I, J, K, L) ; and (3) the intermediate forms, in which 

 what will become the mesorostral bone of Turner is in younger or 

 more advanced stages of growth (C, D, E, F, G, H), — of these Gl- 

 and H are approaching maturity. 



The table on the opposite page gives the principal dimensions of 

 all the more complete specimens measured by me which I attribute 

 to Mesoj^jlodon grayi, Haast. 



On comparing these measurements and the photographs of the 

 crania which I exhibit, it is impossible not to be struck with the 

 similarity of their general outlines — Mesoploclon haasti (L) with the 

 type (K) ; M. av.stralis (Flower) (J) with the Kaiapoi skull (I); 

 and the skull from the skeleton of the co-type in the Canterbury 

 Museum (H) with Van Beneden's figure of M. grayi, Haast. The 

 skull of the younger Otago Museum specimen (A) graduates 

 through the Chatham Island form (D) to the somewhat older 

 representative in the Museum of the Boyal College of Surgeons (C). 

 The known female forms have more gracefully attenuated rostra, 

 the males wanting in this respect somewhat owing to a greater 

 development of the buttress formed by the maxillaries, palatines, 

 and pterygoids. The female rostra are also longer than those of 

 the males, as the measurements show where the sexes have been 

 determined. 



Seen from above. 



The form of the rostrum may be observed (fig. 1, p. 221) in the 

 younger specimens to be wider at the base and less slender 

 throughout its length than in individuals of greater age. The 

 maxillary tubercle has a more sloping and less acute-angled 

 shoulder, and the maxillary bones {mx) are wider, and form, as 

 they emerge on the rostrum, a more prominent flange (or upper 

 border) to the basirostral groove on each side, than in older speci- 

 mens. They run forward on the sides of the rostrum (well seen 

 in Van Beneden's figure of M. graiji) along the premaxillaries as 

 broad bands, one on each side, narrowing and descending towards 

 the inferior surface as they proceed, while with the increasing age 

 of the animal they become narrower and shorter on the sides of 

 the rostrum, thus reducing the length and prominence of the basi- 

 rostral grooves through the disappearance of their superior flanges. 

 The inferior flanges of the groove ai"e more prominent than the 

 superior, extending in A for 1| inch more anteriorly than the 

 superior. In the younger forms the supraoccipital (s.oc) is wide 

 aud flat behind, and its apex in the vertex expands more ante- 



