298 
ME. C. S. TOMES ON THE STEUCTUEE AND 
an investment of enamel, partial or complete, but that cementum is only present in a 
few instances. 
The only reptilian teeth which are really coated with cementum, so far as I am aware, 
are those which are implanted in more or less complete sockets or in a groove. Thus the 
teeth of the Crocodile and of the Ichthyosaurus have cementum upon their basal portions ; 
but whether the inference that cementum is in all cases associated with implantation 
in sockets will be borne out by a more extended series of observations, I cannot as yet 
with certainty say. 
In proof of my statement that the thin outer layer upon Ophidian teeth is enamel, 
the following facts may be advanced : — 
Its refractive index is high, so that it resembles enamel and does not resemble 
cementum. It is very brittle*, so that it is often entirely lost in grinding down a thin 
section, and is invariably much cracked when it does remain in situ (see Plate 48. fig. 1). 
The application of acids to the sections wholly removes it, whereas cementum is even 
less affected by acids than is dentine. 
But what is more conclusive than all is its development ; it is formed from the 
elongated cells of a perfectly characteristic and unmistakable enamel-organ, to be 
presently described — a fact which alone would put it beyond all doubt that enamel 
is present on the teeth of Ophidia, and that therefore cementum is not. 
So far as tlie general plan of formation of individual tooth-germs goes, the teeth 
of Ophidia conform pretty closely with those of Mammalia or Sauria ; but whilst the 
essential points are adhered to, there is so much difference in matters of detail that at 
first sight the sections of the tooth-developing region of a lizard and of a snake are 
strikingly dissimilar. And although it is no more than was to be expected from the 
other characters of the animal, it may be interesting to note that, in respect of the 
development of its teeth, the slowworm is essentially a lizard, and does not show the 
smallest tendency towards that arrangement of the successional tooth-germs which is 
so eminently characteristic of the Ophidia. 
The tooth-germ of a snake consists of a dentine-organ or dentine-papilla (b in 
figs. 4 & 5) (which presents no special characters by which it might be distinguished 
from that of other animals), an enamel-organ, and a feebly developed connective-tissue 
capsule. 
The enamel-organ (f in figs. 4, 5, & 8)] embraces the dentine-papillae in its entire 
length, and consists almost entirely of the elongated cells which constitute the “ enamel 
cells ” or “ internal epithelium of the enamel-organ.” They are nucleated at the 
extremity furthest from the dentine, and closely resemble those of other animals. In 
* Its brittleness did not escape the notice of Prof. Owev (Odontography, p. 22a), who speaks of it as “more 
readily detached from the dentine where it is thickest at the base of the tooth than in other teeth ; portions 
of it adhering to the section are shown at fig. 1 aa, plate 65 whilst a few pages further, speaking of poison- 
fangs, he says, “ from its transparency it has been regarded as enamel. There is, however, no trace of true 
enamel on the teeth of poisonous serpents any more than upon those of the innocuous species.” 
