Microscopic Structure of the Teeth of the Lepidosiet. 361 
prolongations of the pulp cavity, but very few of them coming 
from that cavity itself. 
In the apical portion, which is perfectly smooth externally, the 
internal structure is much more simple than that of the basal por- 
tion which has just been described, there existing no involutions 
of cementum, and no radiations of the pulp cavity. ‘The calci- 
gerous tubes in the apex radiate at once from the central cavity, 
extending towards the circumference. 
The teeth of the Lepidosteus platyrhinus, Raf. (fig. 4,) com- 
monly known as the “duck-bill gar,” are constructed upon the 
same general plan, though more complex in the details. ‘The 
involuted cementum extends in straight lines towards the centre 
for a very short distance only, and then becomes more or less 
irregular in its course, sometimes being undulated, and at others 
changing its direction suddenly so as to form angles, (fig. 4, 6.) 
The pulp cavity and its prolongations are also more or less irregu- 
lar, according to the condition of the cementum, generally ter- 
minating in a simple dilatation, or, as is sometimes the case, bi- 
furcating, as at fig. 4, @. In the central portion of the tooth exist 
also numerous pulp canals, which send off calcigerous tubes from 
their circumference, a conformation similar to that met with in 
the Rhizodus, among the extinct Sauroids. 
Remarks.—In considering the structure of the teeth above de- 
scribed, no one can fail to recognize thé analogy which exists 
between them and those of the Labyrinthodonts, described by 
Prof. Owen in his Odontography ; and had we nothing but the 
teeth of the respective animals to which they belong with which 
to institute comparisons, they would both be referred at once, 
without doubt, to one and the same natural family. Prof. Owen 
does not appear to have been aware. of the existence of the Laby- 
rinthodontic structure in the Lepidostei, since in speaking of the 
teeth of this genus no reference whatever is made to it, and in 
describing the involuted cementum of the Labyrinthodonts he 
says, ‘‘such a disposition of the external substance may be traced 
at the base of the tooth in a few fishes, but is more conspicuous 
in the fang of the Ichthyosaurus.”* If any one will make a 
comparison between the accompanying figure of the tooth of the 
* Odontography, or a Treatise on the Comparative Anatomy of the Teeth, &c., 
by Richard Owen, F.R.S. &c. Vol. I, p. 201. 
Vol. xiv, No. 2.—July—Sept. 1843. 46 
