DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
659 
arranged in rows which are more or less parallel with the mesial line of the fish*. 
Fig. 16 is a still more highly magnified representation of one of these teeth, with the 
basis upon which it is planted. Like the majority of the dermal teeth of fish, it is 
simple, containing an elongated pulp-cavity (fig. 16 a), from which numerous kosmine 
tubes are given off; at its lower extremity (16 h) it suddenly becomes contracted, the 
pulp-cavity being prolonged through the narrowed projecting base (16 c). 
On the surface of the scale there are numerous small circular cavities {\Qd), which 
communicate inferiorly with branches from the network of Haversian canals. Each 
cavityis surrounded by a narrow projecting rim (16e), upon which the flanging shoulders 
of the tooth rest, whilst its constricted base is fitted into the inclosed hole, thus pro- 
ducing an arrangement which closely resembles a ball-and-socket joint, and which 
must allow of a considerable degree of motion in every direction. The tooth is ap- 
parently held in its place by a capsular expansion of the membrane (fig. 16/) which 
covers the surface of the scale. From the direct communication that exists between 
the cavities (16 c?) and the Haversian canals (Idg-T)? il' obvious that the latter 
furnish the pulp-cavity with its nutrient supplies. The one opens directly into the 
other. It only requires the tooth to be fixed instead of moveable, and depressed 
instead of acuminate, in order to render it the exact homologue of one of the areolte 
in the kosmine of Megalichthys. It also reminds us of a very similar connection which 
exists between the ramified dentinal canals of the tooth of the Pike, and the corre- 
sponding ramifications of the Haversian canals in the subjacent bone. 
By studying the structure of the above scale, along with that existing in Cycloid 
and Ctenoid scales, vve are enabled to comprehend what was previously obscure, viz. 
that of the scale of Macropoma Mantelli from the Chalk. It is now obvious that the 
vertical section figured in my last memoir^ reveals a structure which is identical with 
that of a cycloid fish ; only in the process of fossilization, the lowest or membranous 
layer has disappeared, leaving the calcareous laminse of the middle layer, as seen near 
the margin of one of these recent scales. But in the Macropoma, instead of being 
merely covered with a layer of what I have regarded as ganoin, as in the Carp, Perch, 
&.C., this tissue is surmounted by an abundant development of kosmine, in the form 
of large pointed teeth §, which closely resemble those of the Siluroid fish just de- 
scribed. They differ little in the two cases, beyond the fact, that whilst in the Silu- 
roid the teeth have contracted bases and are moveable, in the Macropoma these bases 
are expanded, and firmly fixed upon the upper surface of the scale. We thus see how 
* Some similar teeth have been described and figured by M. Agassiz, as existing on the scale of Hypo- 
stoma placostomus , Poissons Fossiles, vol. i. tab. H. fig. 31 and 32. He speaks of their constricted bases and 
the circular holes into which they are fitted, but refers to the latter as existing in the enamel (email). In 
the present scale we have no such substance ; it is entirely composed of true osseous tissue ; the details of the 
structure upon which the teeth are fixed are not show'n in a very definite manner in the figures. 
t Their continuity does not happen to be shown in the specimen from which the above figure was drawn. 
J Philosophical Transactions, 1849, Part 11. tab. 43, fig. 27. 
§ Ut supra, fig. 28. 
