132 
Proceedings of Royal Society of Edinburgh. [sess. 1 
if constructed on a sufficiently small scale, transmit vibrations of 
light exactly as does the ether of nature. And it would be in- ; 
capable of transmitting condensational-rarefactional waves, because 
it is absolutely devoid of resistance to condensation and rarefaction. 
It is, in fact, a mechanical realisation of the medium to which I 
was led one and a half years ago,* from Green’s original theory, 
by purely optical reasons, in endeavouring to explain results of 
observation regarding the refraction and reflection of light. 
On the Swimming Bladder and Plying Powers of Dadyl- 
opterus volitans. By W. L. Calderwood. Communicated j 
by Professor Ewart. (With Plate.) 
(Read February 17, 1890.) 
Dadylopterus volitans , the so-called “ flying gurnard,” is not in- 
cluded by Gunther in the genus Triglidae, as its name might 
imply, but is assigned to the small allied family of Cataphracti. 
Gunther diagnoses it as follows : — ■“ Dactylopterus, no lateral line, 
pectoral fins very large, an organ of flight, with the upper portion 
detached and shorter, granular teeth in the jaws, none on the palate, 
air bladder divided into two lateral halves, each with a large 
muscle.” 
Before proceeding to describe in detail the swimming bladder 
and other anatomical peculiarities, it may be well to state, that the j 
skull is provided with a superficial bony covering which projects 
backwards over the region of the “ shoulder ” in two flattened 
plates, each terminating in a spine (seen in fig. 2). Also that the 
first four vertebrae of the column have coalesced so as to form a rigid 
tube, the neural spines being united as a vertical plate, which for 
convenience I have termed the neural plate. 
The swimming bladder , so far as I am aware, has a unique 
position, since it is situated not below, but above the vertebral 
column, not forming part of the abdominal contents, but situated 
dorsally in a special cavity of its own. The appearance of the 
* Philosophical Magazine , Nov. 1888, “ On the Reflection and Refraction of 
Light,” by Sir W. Thomson. 
