SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 453 
With the exception of the ganoin and kosmine, all the various modifications of 
laminae in this scale abound in lacunae. The bony matter surrounding the Haversian 
canals is deposited in concentric lamellae, between which are numerous lacunae with 
their stellate canaliculi of various forms, usually of the common icbthyal type; but 
sometimes, especially in the lamellae nearest to the canals, showing a disposition to 
become elongated in the direction parallel to the axis of the latter. In the hori- 
zontal laminae, already described, fig. 15 /, we find a very curious form of lacuna. 
They are very much elongated, being about inch in length, and some- 
times almost linear, giving off numerous rectangular canaliculi. Not unfrequently 
these spring from the lacuna diagonally; those on different sides verging to the 
opposite extremities of the lacuna, as is seen in fig. \9h, which sketch represents 
the appearance of these lacunae as seen under a magnifier of 300 diameters linear. 
Those of each layer exhibit a considerable tendency to parallelism of arrangement, 
but owing to the extreme thinness of the laminae, the lacunae belonging to two 
or three layers may be seen at once, even under a magnifying power of 300, as 
shown in fig. 19: and it is a curious fact, that those constituting one layer exhibit a 
very considerable tendency to run in the direction of the canaliculi of an adjoining 
layer. In fig. 19, u, h and c represent individual lacunae belonging to three of these 
parallel series. In addition to the above, each lacuna gives off small vertical cana- 
liculi, which penetrate the lamellae, and thus connect the different layers together, 
fig. 15/. These structures become highly interesting when viewed in connection 
with Mr. Quekett’s instructive attempts to identify the bones of the four classes of 
the Vertebrata by means of the variations of their microscopic structure*. In that 
memoir Mr. Quekett considers that this elongated form of lacuna is characteristic 
of the Reptilia; and there is certainly a very striking resemblance between his repre- 
sentation of those of the Pterodactyle-f-, the accuracy of which, my own specimens of 
the latter confirm, and my fig. 19. We thus find that some of the elementary tissues 
of this fish, which on its first discovery was so readily mistaken for a reptile, exhibit 
a most striking resemblance to the reptilian type: I shall have to show, by and by, 
that the same form of lacuna exists in the genera Diplopterus and Holoptychius ; 
consequently the fusiform lacuna can no longer be regarded as typical of the Reptilia, 
as was imagined by Mr. Quekett, though it is unquestionably the form most 
commonly found in that class of Vertebrata, as the quadrate one is chiefly character- 
istic of fish : great caution however requires to be exhibited ere we decide a disputed 
question on this evidence alone. I find but little difference between the majority of 
the lacunae of the small Platysomus parvulus, already described, the scales of which are 
about the ^th of an inch in length, and those from the gigantic femur of an Iguanodon, 
in the possession of Dr. Mantell, which, when perfect, he informs me has not been 
less than 27 inches in circumference at the shaft. A legitimate inference from these 
* Transactions of the Microscopical Society of London, vol. ii. part 2. p. 46. 
t Tab. 8. fig. 2, ut supra. 
3 N 
MDCCCXLIX. 
