Ray Lankester, on the Tooth in Ziphius Sowerbiensis. 57 
The lower jaw exhibits two large teeth very deeply im- 
planted in their sockets, of a curved or somewhat claw-like 
form, very much compressed laterally, and fixed so that the 
curve is directed backwards instead of forwards, as is usual 
with teeth having the general form of canines. 
External characters of the tooth. — In Pl. V, fig. 1, one of 
the teeth is drawn of the natural size ; the line a, b } indicates 
the direction of the horizontal line of the alveolus when the 
tooth is in its natural position in the jaw. The lateral com- 
pression of the tooth is seen from fig. 3, which is half of a 
transverse section taken at a. The whole exterior surface, 
with the exception of the small conical crown, is very rough, 
irregular, and knotted, having a somewhat resinous lustre 
and a yellowish colour. This external tissue is, as in other 
teeth, “ cement.” The small conical crown which seems to 
rise out of the cement like a nipple is in one of the two 
teeth very sharply marked off from the rest of the tooth by 
the presence of a crack between it and the yellow nodu- 
liferous cement. Its surface has the peculiar vitreous lustre 
characteristic of dentine, and is white. That tooth which has 
not been cut is heavy for its size, and appears to be solid and 
compact throughout.* 
Longitudinal section of the tooth. — In PI. V, fig. 2, the 
longitudinal section of the tooth is drawn to a certain extent 
diagrammatically, the small remnant of a pulp-cavity being 
introduced from the opposite half of the section. The true 
nature of the various layers exhibited was ascertained by 
microscopical examination of three cross sections cut at about 
the point a , and one sixth of an inch lower (PI. V, fig. 1, 
and PI. Y, fig. 3) from the other half of the specimen. 
The great thickness of cement is very remarkable (c), 
completely enveloping the small cap of true dentine (d), 
with the exception of its projecting point. The cement 
forms the outer wall of all the rest of the tooth, being con- 
tinued round the calcified pulp-cavity, into which it projects 
very largely in a curiously irregular manner in some places. 
The small cap of dentine (d) is very clearly marked off 
from the rest of the tooth in the section by the bri g ht 
vitreous lustre it has acquired in the polishing to which the 
specimen has been submitted. This small conical cap, hardly 
more than half an inch in length, appears to be all the true 
dentine developed in the tooth, with the exception of a very 
thin layer extending between the cement and calcified pulp 
about halfway down the tooth, which is, however, for the 
most part composed of a structureless “ globular ” mass of 
* Sowerby in his description characterises the teeth as “honey.” 
