422 



PEOr. W. H. PLOWEE ON THE GENUS MBSOPLODON. 



stated) is given by Dr. Hector as 15' 6" ; this difference, however, can scarcely be con- 

 sidered beyond the limits of individual variation, b. 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 M. 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 M. grayi this pit is quite shallow. It is possible, 

 however, that this may be a question of age. 



Pig. 1. 



Fig. 2. 



Mesoplodon australis (adult). 



Mesoplodon grayi (young). 



Pig. 3. 



Fig. -1. 



Mesoplodon haasii (adult). Mesoplodon hec'ori (young). 



OutUues 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 M. 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 22^", while that of the adult 

 M. australis is only 18". Making eveiy allowance for individual variation, it scarcely 

 seems possible that a rostrum such as that shown in fig. 2 could change in the course 



