360. Microscopic Structure of the Teeth of the Lepidostei. 
or less striated or fluted near the basal portion. The base is im~ 
planted in a cavity or alveolus, with which it is anchylosed ; but 
from which, when shed, the teeth are detached by the absorption 
of their substance at the points of union. 
The teeth of the Lepidosteus platyrhinus, Raf. and L. oxy- 
urus, Raf. when cut at right angles to their axes, present a sur 
face which is subdivided into numerous segments, by lines ex- 
tending from the circumference towards the centre, the whole’ 
resembling somewhat a section of a porcupine quill. Under the 
microscope this appearance is seen to be the result of a peculiar- 
ity of organization hitherto undescribed in the class of fishes, and 
to which there is an approximation in the teeth of the Ichthyo- 
saurus, and a still closer resemblance in those of the Labyrintho- 
donts. With regard to the teeth of the Lepidosteus feror, Raf. 
Iam unable to give any information, since as yet I have not been 
able to submit them to microscopic examination. Fae 
In the Lepidosteus oryurus, where the teeth are the least com~) 
plicated, (though constructed upon the same plan as in the L. pla- 
tyrhinus,) the basal portion is fluted, while that near the apex is 
perfectly smooth, which differences correspond to others in the 
substance of the tooth still more striking. (See fig. 1, f.) Ex 
ternally the tooth is covered with a layer of investing substance 
or “cementum,” (figs. 1 and 2, 6,) which follows the outline 
of the more internal fluted portions; and within this is a second 
more transparent layer, (figs. 1 and 2, ¢,) which at the space be- 
tween two adjoining convolutions extends in a straight or but 
slightly curved line. towards the centre, (fig. 2, d,) and being 
folded on itself returns again towards the circumference; and 
this is repeated between all the different segments. The length 
of the involved. portion is equal to a little more’ than one third of 
the diameter of the tooth. The pulp cavity, which in the ma 
ture tooth is quite small, is charactérized by the existence of ra- 
diations from its circumference towards the exterior of the tooth, 
terminating at equal distances from its centre, and occupying the 
middle of the spaces between the involutions of the cementum, 
(figs. land 2, ¢.) The space comprised between the involutions 
_ of cementum and the radiations from the pulp cavity, is filled. 
“with “dentine,” which is characterized by the existence of calei~ 
gerous tubes, of which there are highly magnified representations 
in fig. 2, a, and 3; these last in nearly all cases radiate from the. 
