610 BRITISH PALZOZOIC FOSSILS. [ Pisces. 
Genus. MEGALICHTHYS (Ag.) 
(For synonyms, see Lhizodus Hibberti). 
Mercauicutuys Hreperti (Ay.) 
Ref. and Syn.—Ag. Poiss. Foss. Vol. II. t. 63, 64. 
The only examples of this genus (as defined in the Poissons Fossiles of Agassiz) are a few plates of 
the head, and some large quadrate scales measuring about three-quarters of an inch across. Both these parts 
are thick and bony, covered with an unusually thick lustrous coat of ganoine, the surface of which is very 
minutely and closely punctured under the lens, and many of them have, in addition, several much larger, 
irregularly scattered minute pits (like those produced by the point of a pin). The scales are remarkable for 
having the ganoine separated from the concealed part of the scale at the posterior and superior edges by a 
deep groove, into which a corresponding marginal ridge of the adjoining scale fits. With these scales 
are mingled numerous others of about half their size, or less, and with the articular sulcus nearly obsolete ; 
these seem to belong to the manuscript species Diplopterus carbonarius of (Ag.), and one small portion of the 
posterior end of the fish, shewing some portion of the fins, seems to favour this generic reference, although the 
medial prolongation of the body is much shorter than in the Devonian species, as in Carlacanthus, but with two 
remote dorsals; at the same time it is impossible to avoid suspecting that they both belong to the same fish. 
I greatly regret, however, that the meagreness and imperfection of the specimens at my disposal, compared 
with those which are understood to exist in other collections, prevent me coming to any definite opinion on 
the point. 
Position and Locality—Carboniferous limestone of the coal-measures at Burdiehouse, near Edinburgh, 
and Shott’s Iron-works, Lanarkshire. 
Genus. CENTRODUS (dfCoy). 
(Etym. xévrpov, galli calcar, and édéots, dens.) 
Ref—Id. M°Coy, Ann. Nat. Hist. 2nd Series, Vol. IT. 
Gen. Char.—Tooth simply conical, gradually tapering, slightly curved backwards, apex pointed; section 
circular throughout ; medullary cavity large, conical, simple, so wide at base that the tooth is reduced to a thin 
edge ; surface even, very finely striated longitudinally. Microscopie structure :—exceedingly fine calcigerous 
tubes radiating directly from the pulp-cavity towards the periphery, near which they terminate in numerous 
very minute calcigerous cells, beyond which is a narrow clear layer, bounded by a definite dark line, outside of 
which is a coating of glass-like enamel, without perceptible organic structure under a magnifying power of 
three-hundred diameters. 
Externally these teeth slightly resemble Rhizodus (Ow.), and lately I have seen a slight trace of 
plication at the base, as in that genus, but they are distinguished by their circular section, and very wide, 
simple pulp-cavity, which latter distinguishes it from nearly all paleeozoic teeth except the Old Red sandstone 
genus Cricodus (Ag.), from which it is known by its more slender conical form, and wanting the strong 
longitudinal ridges of the surface, as well as by possessing the distinctly defined enamel layer, which has 
all the appearance of having been secreted by a distinct organ, and is quite different from the condensed 
dentine which forms the false enamel or ganoine of most fish-teeth. Since the above description was written, I 
have seen a similar if not identical tooth attached to a head closely resembling the Leeds fish figured as 
Megalichthys Hibberti by Agassiz, in the Bristol Museum; but having no opportunity of examining the matter 
further, I can only throw out the hint for future observers to follow up. The teeth of Megalichthys, according 
to Agassiz, are minute and uniform, and haye no resemblance to the present type. 
