BRITISH rOSSIL CETACEA 



Species — Zirnius medilineatus. 



Zii'iiius MEDILINEATUS, Owen. Plate IV, fig. 3. 



In the by no means easy task of choosing a name from specific characters shown by 

 this class of fossils, I have been led to the one above given by the extent to which such 



mid-line, indicative of a bipartition of the mid- 

 tract (14', 14'), extends along that tract in the 

 present specimen. It is present in a feebler 

 degree and for a less extent in another specimen, 

 afterwards to be described, which may indicate a 

 variety of the present species, but such character 

 of the mid-tract of this species is not the only 

 one by which it differs from the foregoing Ziphii. 



With reference to the character of the largest 

 and first-described specimen, suggesting for it 

 the name planus, it might be objected that a 

 convexity of the mid-tract, as shown in Z. gibbus, 

 had been worn down by posthumous abrasion. 

 So, likewise, it may be said of the present 

 species, that the persistence of the median line 

 ^ . , ^. , . , ,. is due to the nonage of the individual. But, 



Section of rostrum, Zi^AiM* merfj/enea^tt*. " 



if so, it coexists with a flatness of the part of the 

 prefrontal mid-tract along which it extends that cannot be the result of abrasion, for the 

 sides of the longitudinal fissure are convex ; and yet the degree of convexity is so slight 

 on each side of the fissure that the mid-tract gives as flattened a character of the upper 

 posterior part of the snout as in Ziphius planus. I conclude, therefore, that the difference 

 in the character of the mid-tract between the present species and Ziphius angustus, Z. 

 angulatus, Z. gibbus, to be due to an original and inherent structural specific character of 

 the skull in Ziphius medilineatus. From Ziphius planus it difiers not only in size, but in 

 the greater degree of transverse convexity of the upper part of the snout between the fore 

 parts of the ectolateral ridges e, e (as, e.g., at the place, *, of the section figured below fig. 3, 

 PI. IV), and in the greater relative longitudinal extent of those ridges ; there is not, besides, 

 any trace in Z. planus of the median linear groove characteristic of the prefrontal tract (i4', 14', 

 fig. 3) in the present species. The short oblique irregular fissure on the outside of 14, in 

 fig. 3, simulates a suture defining the prefrontal from the mid-tract, as the mid-line along that 



