422 PROF. W. H. FLOWER ON THE GENUS MESOPLODON. 
stated) is given by Dr. Hector as 15’ 6"; this difference, however, can scarcely be con- 
sidered beyond the limits of individual variation. 6. The form of the tympanic bone 
differs slightly, the inner side of the posterior edge (as seen from below) being more 
bevelled and the inner posterior lobe less prominent in WM. australis. c. The lateral 
groove at the base of the rostrum is very much deeper in M. australis, running up- 
wards into the maxillary bone, and forming a blind pit at its upper termination, of the 
depth of nearly one inch, whereas in MW. grayi this pit is quite shallow. It is possible, 
however, that this may be a question of age. 
Fig. 1. Fig. 2. 
Mesoplodon australis (adult). Mesoplodon grayi (young), 
Fig. 3. Fig. 4. 
Mesoplodon haasti (adult). Mesoplodon hectori (young). 
Outlines of section of middle of rostrum. Natural size. 
While the section of the rostrum of both the above (figs. 1 & 2) agree very closely 
in general outline and size, that of the supposed VW. haasti (fig. 3) differs very greatly, 
being altogether more compressed and wanting the lateral wing-like ridges near the 
lower border ; its length is also considerably greater, being 221”, while that of the adult 
M. australis is only 18". Making every allowance for individual variation, it scarcely 
seems possible that a rostrum such as that shown in fig. 2 could change in the course 
