226 MB. H. o. FORBES ON THE [Feb. 28, 



easy of explanation. The gibbosities, it will be observed, occur 

 n the rostrum over those regions where the vomer does not 

 reach the palatal surface. The removal of the wedge gives more 

 space beneath, causing the premaxillaries to gape, while more 

 anteriorly, in front of the place where the vomer vanishes, the 

 premaxillaries stand still more apart. In the regions where the 

 edges of the premaxillaries are closest the vomer is wedged in on the 

 palatal surface between the bases of the maxillaries, and there the 

 rostral bone, as a rule, grows densest and protrudes farthest above 

 the level of the premaxillaries, and just there often sho\^s no median 

 line or suture. In the male Z. cavirosiris the greatest growth of the 

 mesorostral occurs where not only the vomer, but also the lower 

 edges of the premaxillaries, protrude on the palatal surface. 



The lines or sutures on the surface of the mesorostral are pro- 

 duced by various causes, sometimes (as in the specimen, Plate 

 Xlll. figs. 1, 1 a, v.s) by the two wings of the vomer meeting in tlie 

 centre \ when the suture may persist or may become lost, according 

 to the amount of squeezing the mesorostral undergoes. Then on 

 each side of the solidified vomer may appear the sutures of the 

 premaxillaries (Plate XIII. fig. 1 a, pmx.s), and very often the 

 thickenings of the interior surface of the premaxillaries (^jm.i'.o) 

 grow up between and shoot above the original petrous walls of 

 these bones, forming another suture, so that there may be as many 

 as five lines traceable on the surface. There may be more if, as 

 sometimes occurs, one of these segments becomes crumpled (Plate 

 XIII. tig. 1, Plate XIV. fig. 5,vir.cr) in the general squeeze of the 

 parts. Hence, as diagnostic characters (cf. M. medilineatus), the 

 lines on the mesorostral bone are also quite valueless. In the 

 most anterior part of the rostrum there is only one median suture 

 (pmx.s), often very well marked, especially in old individuals, where 

 the osseous growths on the interior surfaces of the premaxillaries 

 meet. As has been pointed out both by Sir W. Flower and Sir W. 

 Turner, a suture, or often a deep depression, between the meseth- 

 moid and the mesorostral is generally visible (Plate XII. fig. 1, 

 Plate XIII, fig, 1, meth.s). 



On comparing the different specimens which I have had an 

 opportunity of examining personally, or by their various published 

 descriptions, the species of the genus Mesoplodon seem to me 

 to be reducible to six : — 



1. Mesoplodon bidexs (Sowerby). 



Cf. Flower, Trans. Zool. Soc. x. p. 415 (1878), 



2, Mesoplodon eubop.eus (Gervais). 



Cf. Flower, Trans. Zool. Soc. x. p, 416 (1878), 



^ " It seems probable (as Duvernoy has already pointed out) that the ' central 

 area' indicates the upper extent of the vomer, the only remains of the 

 primitive trough-like cavity being the median slit above unci the large fossa 

 behind." — Huxley, Quart. Journ. Geol. Soc. vol. xx. p. 394 (1864). 



