SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 467 
to the ganoid scales, indicate a series of analogies which can scarcely be questioned. 
But I should even venture to go still further ; I can trace no real difference between 
the tubercles of the scale of Dapidius granulosus, and what I have called the kos- 
mine, in Lepidotus and its allies. The latter appears to me to be only a more ex- 
tended development of the former. In the beautiful form in which this kosmine 
exists on the scales of MegaUcJitliys, Dlplopterus and Holoptychius, we have nothing 
more, apparently, than the confluent aggregation and superficial depression of a 
number of placoid teeth, surmounting a highly developed bony scale. Compare for a 
moment the horizontal section of the shagreen of the Dog-fish, fig. 31, with the hori- 
zontal section of the Dlplopterus, fig. 20. The dermal teeth of the former are repre- 
sented by the areolae of the latter ; the pulp-cavity and branching tubes of the true 
dermal tooth, fig. 31 Z), have their homologues in the ascending central cavities and 
branching tubes of the areolae of the Dlplopterus, fig. 20 m. In the same way the 
ascending cul-de-sac in each areola of the MegaUchthys appears to correspond with 
the pulp-cavity, whilst the arborescent tubidi which it gives off represent the dentine- 
like tubes of the shagreen. The chief difference appears to consist in the fact, that 
in the Ganoid, the areolae, being closely aggregated upon a bony basis, have coalesced, 
and been flattened, superiorly, to an uniform level ; whilst in the Placoid, each areola 
forms an isolated conical tooth, implanted in the soft integument. The different de- 
grees to which the same structure may be either flattened, or drawn out and become 
acuminate, is seen in the various parts of the exo-skeleton of Macropoma, showing 
that the process is a very trivial one, involving no typical change*. 
* Since the above remarks were jjenned, I have found a still more beautiful illustration of this homology. 
When the smooth shining membrane covering the snout of the Saw-fish is examined under the microscope, it 
is found to consist of a thin soft skin, in which are implanted numerous flattened dermal teeth, each resembling, 
in its form, the small studs commonly worn as breast-ornaments. They are packed closely together, with only 
a few minute intervening spaces. This closely aggregate arrangement, combined with their depressed form, 
causes the whole to present a smooth, shining surface, nearly resembling that of a ganoid scale. Fig. 34 re- 
presents a vertical section of some of these teeth with the subjacent tissues ; 34 a is the upper portion of the 
osseous (?) structure of the snout; b, the soft integument; c, individual teeth; d, the pulp-cavity; e, canals 
radiating from the latter, from four to eight existing in each tooth, and arranged as in Macropoma, fig. 25 ; 
/, descending canal, communicating between the pulp-cavity and the subjacent soft integument ; g, dentine- 
like (kosmine) tubes ; h, open spaces surrounding each tooth, and appearing, when viewed vertically by trans- 
mitted light, like a network of canals, reminding us most forcibly of the similar appearance surrounding each 
areola 'm MegaUchthys, fig. 16. Where three or four of the teeth meet there is usually a minute space not 
filled up, opening into this network, which latter is formed by the horizontal constriction of the teeth, as seen 
in 34 which represents the exterior of an individual which the section has not divided. 
On comparing this section with the vertical one of MegaUchthys, fig. 15, the homologies of the various parts 
are still more striking than in the example of the Dog-fish. Each tooth in the Saw-fish represents one superficial 
areola of the MegaUchthys. The small superficial intervals between the teeth appear to be the homologues of 
the descending trumpet-shaped cavities, 15 a; these communicate between the exterior and the interdental 
spaces, 34 h, which apparently correspond with the network of small tubes in MegaUchthys, 15 c and 16 c, as 
already observed. The pulp-cavity, 34 d, takes the place of the cul-de-sac, 15 f, a communication being main- 
tained between the interdental spaces and these pulp-cavities, by means of the radiating canals, 34 e. And, as 
