460 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
comparing figs. 15 and 24 together, and bearing in mind the concentric lines in the 
kosmine of the former, this identity will be obvious at a glance ; and it will be seen 
how the addition of successive lamellm to such a framework as fig. 24 exhibits, would 
lead to all the results which we find in Megalichthys, and also establishes the close 
connection that exists between the two genera, as well as between them and Diplo- 
pterus; instead of one being found amongst the Coelacanths, and the other two amongst 
the Sauroids, this resemblance, connected with the close analogy existing between 
their teeth and such fragments of bone as have been met with, requires that they 
should in future be classed side by side. 
No doubt can exist that in these species of Holoptychius, the bony lamellae have 
been deposited on the same plan that we have found to prevail throughout all the 
forms of scale which I have examined. This is especially seen in fig. 23. The exist- 
ence of the Haversian canals can be distinctly traced, either to the inflexions of 
these lamellae, or to the leaving out of portions of them, as in the case of the vertical 
branches. 
Bearing in mind the close affinity just noticed, between the genus under consi- 
deration and Megalichthys, we can scarcely suppose it probable that their scales have 
been constructed on two widely different physiological plans. That of Holoptychius 
appears to be intermediate, as to the complexity of its structure, between those of 
Acipenser and Megalichthys ; consequently we can scarcely resist the conclusion, to 
which the study of the latter fish alone has led me to incline, that complicated as its 
scales are, they have been formed, ab initio, on the same plan of intramembranous 
ossification as all the rest. 
Judging from the descriptions given by M. Agassiz, it appears evident that a re- 
cent example of a scale somewhat similar to the type found in Holoptychius, occurs 
in the Polypterus of the Nile. Though in his description M. Agassiz does not notice 
anything analogous to the forms of kosmine described in the last genera, yet in the 
horizontal network of canals, and their vertical branches communicating with both 
the upper and lower surfaces, we have an analogy too evident to be overlooked ; and 
one which attracted the attention of the Swiss philosopher, whilst examining some 
of the Diplopteri from the old red sandstone. It is highly interesting to find, that, 
though we have so small a number of ganoid fish still existing, when compared with 
the multitudes which crowded the ancient seas of our globe, we have, in the Bony 
Pike, the Sturgeon and the Polypterus, living representatives of the most conspicuous 
types of scale-structure found amongst their fossil allies. 
Macropoma. — This anomalous genus has long been a source of perplexity to 
ichthyologists. Macropoma Mantelli, first discovered in the Sussex chalk by the 
distinguished geologist whose name it bears, was first arranged by M. Agassiz 
amongst the sauroid subdivision of the ganoid fish. He afterwards removed it to the 
Coelacanths, and still more recently he has proposed to unite it with the genus Undina 
of Munster and some others, of which he designed to form a new group*. On 
% 
* Poissons Fossiles de Vieux Gres Rouge, p. 61. 
