444 MR. VF. c. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
all exhibit complete circles, those forming the canal alone doing so ; on one side of, and . 
external to the latter, we often observe a number of half-circles, as if it had for a long 
time been merely a groove at the margin of the scale. In other respects, the bony 
lamellae of the operculum exhibit the same appearances as those of the true scales, 
and prove that it has been formed by a similar deposition of laminae. This close 
resemblance between the minute structure of the scales and opercular bones of Lepi- 
dotus seems to support the view once entertained by Prof. Owen, but since abandoned 
by him, that the opercular bone is merely a modified scale, and consequently belongs 
not to the endo- but to the exo-skeleton. 
Lepidotus Mantelli and L.Jimhriatus have scales of a similar structure to those of 
L. sefniserratiis, only the latter appears to want the beautiful ramifications of the 
kosmine. The central tubes which ascend from below all terminate in short branches 
like those seen in the centre of the last-described scale. The ganoin of L. Mantelli 
is filled with minute brown granular points ; but whether these are parts of its original 
structure, or whether it is merely an effect of fossilization, I am undecided. I suspect 
the former to be the case. Similar, but still more minute, granules exist in great 
abundance in the ganoin of L.Jimhriatus. 
Semhiotus. — Fig. 9 represents a vertical section of Seminotus rhombijer, taken 
parallel to the mesial line of the fish. It exhibits an excellent illustration of the 
general contour of this class of fossil scales : a is the anterior extremity, which is im- 
bedded in the soft skin, its oblique margin being overlapped by the scale in front of 
it : b is the opposite edge, adapted for resting on the anterior bevelled portion of the 
adjoining scale. We have the same arrangement of laminae, lamellae, lacunae, canals 
and lepidine tubes as in Lepidotus, only we want the arborescent ramifications of the 
kosmine tubes. The ganoin exhibits the laminated structure found in the other 
scales. 
Pholidotus. — In PJioUdotus Leachii we have a close resemblance to the scale of 
Seminotus rhomhifer, only the edges of the upturned laminae, as seen in a vertical 
section, exhibit less of a tooth-like arrangement ; and amongst the lower laminae of 
each scale is developed a large central lenticular cavity, produced by the divergence 
of some of the last-formed layers. I have not yet discovered any traces of an open- 
ing into this cavity, though there most probably is one. The parallel tubes take their 
rise from it as from the bases of ordinary scales, ascending towards the ganoin. 
Ptycholepis Bollensis. — In the structure of the small thick scale of this curious spe- 
cies, we find a resemblance to that of Lepidotus semiserratus. The parallel ascending 
canals terminate in a similar thin layer of kosmine, which exhibits three or four 
parallel rows of anastomosing loops, giving off minute branching tubuli. The ganoin, 
which is unusually thick, exhibits precisely the same laminated structure as that of 
the Lepidotus. 
Beryx. — The scales of a new species of this genus, from the Chalk of Sussex, belong 
to the same group. We find the large canals, like those of the Lepidosteus, commu- 
