24 



BRITISH FOSSIL CETACEA 



Species — Ziphius tenuirostrts, Owen. Plate V, figs. 1 and 2. 



The first, or basal, five inches of the prefrontal mid-tract in this specimen is more 

 convex than in Ziphius planus or Z. mcdiUneatus, but is less convex than in Z. yibhus or 

 Z. angustiis} Beyond that point (*, fig. 1) the mid-tract rises with a well-marked trans- 

 versely convex surface, and with a slight longitudinal convexity, after which it is con- 

 tinued straight to the fractured fore end of the snout, preserving its transverse breadth and 

 convexity, as well as the lateral linear grooves or sutural indications of distinction from 

 the side-plates of the premaxillaries ('22). These plates descend from those sutures almost 

 vertically, with a slight outward expansion, to join or coalesce with the maxillaries (21) 

 in the extent of the snout preserved anterior to the ectomaxillary ridges. These ridges 

 (PI. V, figs. 1 and 2, e, e), when in advance of the intraorbital expansion of the maxil- 

 laries, are continued forward for ten inches before final subsidence, the decrease of breadth 

 being very gradual. At about eight inches from their origin the nervo-vascular canal, of 

 which the ridge e at first forms the outer wall, emerges, the ridge itself being then continued 

 onward as the lower border of that canal. The high position of the basal half of the ridges 

 makes them seem to terminate laterally the upper surface of that part of the rostrum, the 

 transverse contour of which upper surface is sinuous, moderately convex at the middle, and 

 concave on each side, as shown in the section at 14', 14', and e, e', PI. V, but in a minor 

 degree than at the more basal parts of the rostrum. The side of the rostrum below the 

 ectomaxillary ridges swells into a longitudinal convex tract ( m, fig. 1, PI. V). The 

 vomer, emerging upon the palate, about half way from the inner end, of the present specimen 

 (fig. 1, 13) contributes about half an inch of its transverse extent to the fractured fore part, 

 indicating, according to the analogy of Ziphius Layardi (PI. I, fig. 3) that this long and 

 slender form of snout must have extended about six inches beyond the fractured fore part 

 of the specimen above described. 



1 Compare the section at the point *, in fig. 1, PI. V, in regard to the convexity of \rith that 



part in the section below fig. 3, PI. IV, or the sections given in PI. III. 



