464 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
endo-skeleton. These lacunae not only distribute their large canaliculi in the plane 
of the lamellae, but shorter vertical twigs penetrate the lamellae, and thus keep up a 
communication between the inner and outer surfaces of the viscus. 
Some of the external lamellae lose their exact parallelism with those below, and 
one in particular assumes an undulatory arrangement, fig. 30 c, in the folds of which, 
alternately above and below, are placed large irregularly-shaped lacunae, fig. 30 d, 
the distorted prolongations from which are obviously modified canaliculi. This 
curious structure is covered over with other more dense and apparently structureless 
lamellae, which fill up the inequalities and restore the parallelism of the surface with 
the lower lamellae, constituting the exterior of the viscus. 
On examining a horizontal preparation of a fragment of the same viscus, we see 
that the undulations of the lamella, fig. 30 c, produce the appearance exhibited by 
fig. 29. Numerous parallel lines enclose corresponding spaces, about the T^o^th of 
an inch in width, which circumscribe the viscus at right angles to its longer diameter. 
The lacunae, fig. 29 a, which are arranged in corresponding rows, are alternately 
above and below the lamella, their irregularly projecting canaliculi, 29 h, giving 
them the aspect of Hebrew or Arabic characters. On making a horizontal section 
amongst the lower lamellae, 30 a, we find that the numerous lacunae are of the com- 
mon ichthyal type, only they are more than usually crowded together, as well as 
anastomose more freely through their spider-like lacunae. No canals of any kind 
pass through the tissue. These facts of course do away with all probability of this ano- 
malous viscus having been a stomacb ; according to Dr. Mantell, to whom we are 
indebted for the discovery of this singular creature, the broad anterior extremity of 
the cylinder is always open, and situated opposite the posterior margin of the oper- 
cular bone, whilst its caudal termination is as invariably closed. 
I am disposed to believe that it has been an organ fulfilling the functions of an 
air-bladder. Its osseous structure would render it capable of resisting a considerable 
amount of pressure, and if its patulous extremity has been closed up by an elastic 
membranous appendage, capable of acting as a valve, this would enable the creature 
to regulate its buoyancy by increasing or diminishing the compression of the con- 
tained air, and thus facilitate its movements in either shallow water or at great 
depths. Except in cases of diseased ossification, the existence of an internal thoracic 
or abdominal viscus, having hard parietes of true bone, is an anomaly, which, as far 
as I am aware, has hitherto presented no parallel in nature. 
The structure of the scale of Macropoma, as now described, is wholly different 
from that presented by any of the ganoid fish noticed in the preceding pages. It 
bears a much closer resemblance in its leading points to the dermal appendages 
found amongst the group of true Placoids, between which and the Ganoids the 
Macropoma appears to form an inosculating link. In order to illustrate this opinion, 
I have accompanied the memoir by figures of portions of the dermal appendages of 
