SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 465 
one or two Placoids, which will enable us better to comprehend the structure and 
affinities of this singular creature. 
Fig-. 31 represents a thin horizontal section of the shagreen or skin of the Dog-fish. 
It consists, as is well known, of a number of small dermal teeth, implanted in a 
more or less linear manner in a soft skin, figs. 31 a, ‘62 a. Each tooth contains a 
pulp-cavity, fig. 31 b, from which radiate several large canals. One of these descends 
vertically, as seen in fig. 32 b, which represents a vertical section of an individual 
tooth. The remainder, varying in number from one to three or four, proceed in a 
horizontal direction towards the posterior portion of the tooth, figs. 31 c, 32 c, where 
they appear to communicate with the most superficial layers of the integument, if 
not with the external surface itself, being apparently the analogues of the radiating 
canals in the tubercles and teeth of Macropoma, figs. 25 c, 26 e, 27 d and 28 
From each pulp-cavity is also given off numerous branching tubes like those seen 
in the dentine of the teeth of sharks. They only differ from those in the correspond- 
ing dermal teeth of Macropoma in being larger and less crowded together. 
The superficial portion of each of these appendages, which is not imbedded in 
the soft cutaneous tissues, is covered over with a very thin layer of glossy 
ganoin, but between which and the tubular structure there is no distinct line of 
demarcation. 
If we compare these vertical and horizontal sections of the dermal teeth of the Dog- 
fish, figs. 3 1 and 32, with the corresponding representations of the operculum and scale 
oi Macropoma, figs. 25, 26 and 28, we cannot fail to be struck with their identity in 
every respect. The only real difference appears to be, that, whilst in the Dog-fish the 
teeth are isolated, being implanted in a soft integument, in the Macropoma they are 
fixed upon a calcareous basis. In the case of the operculum, this basis consists of a 
true osseous structure ; and in the scales, though the true bony matter has dwindled 
down into the thin superficial film surrounding the bases of the teeth, its place is 
supplied by a thin laminated tissue which is its equivalent, as a solid foundation on 
which numerous teeth are aggregated, and which is probably but the homologue of 
the thin laminae of which the stellate bases of the dermal teeth of many Placoids are 
composed. 
If these are true analogies and not mere resemblances, they afford us an interest- 
ing illustration of the successive steps in the development of the hard cutaneous 
covering seen in the ganoid fish : but before endeavouring to trace this development, 
I would direct attention to an additional link in the chain supplied by the fossil 
shagreen of the Hybodus reticulatus, from the lias of Lyme Regis ; a vertical section 
of which is represented in fig. 33. 
We here find another modification of the dermal teeth, fig. 33 a, with large pulp- 
cavities, 33 b, and canals opening laterally as well as vertically, 33 c, communicating 
with the soft tissues in which the teeth have been originally implanted. From these 
pulp-cavities, also radiate branching tubes resembling those of dentine. So far, there 
