432 
MESSRS. F. M. BALFOUR AND W. N. PARKER ON THE 
To the distinctive set of characters given by Muller we may probably add the 
following :— 
(1.) Oviducts and urinary ducts always unite, and open by a common urinogenital 
aperture behind the anus. 
(2.) Skull hyostylic. 
(3.) Segmentation complete in the types so far investigated, though perhaps Amici 
may be found to resemble the Teleostei in this particular. 
(5.) A pronephros of the Teleostean type present in the larva. 
(6.) Thalamencephalon very large and well developed. 
(7.) The ventricle in the posterior part of the cerebrum is not divided behind into 
lateral halves, the roof of the undivided part being extremely thin. 
(8.) Abdominal pores always present. 
The great number of characters just given are amply sufficient to differentiate the 
Ganoids as a group; but, curiously enough, the only characters amongst the whole 
series which have been given, which can be regarded as peculiar to the Ganoids, are 
(1) the characters of the brain, and (2) the fact of the oviducts and kidney ducts 
uniting together and opening by a common pore to the exterior. 
This absence of characters peculiar to the Ganoids is an indication of how widely 
separated in organisation are the different members of this great group. 
At the same time, the only group with which existing Ganoids have close affinities 
is the Teleostei. The points they have in common with the Elasmobranchii are 
merely such as are due to the fact that both retain numerous primitive Vertebrate 
characters,* and the gulf which really separates them is very wide. 
There is again no indication of any close affinity between the Dipnoi and, at any 
rate, existing Ganoids. 
Like the Ganoids, the Dipnoi are no doubt remnants of a very primitive stock; but 
in the conversion of the air-bladder into a true lung, the highly specialised character 
of their limbs,t their peculiar autostylic skulls, the fact of their ventral nasal openings 
leading directly into the mouth, their multisegmented bars (interspinous bars), 
directly prolonged from the neural and haemal arches and supporting the fin-rays of 
the unpaired dorsal and ventral fins, and their well-developed cerebral hemispheres, 
very unlike those of Ganoids and approaching the Amphibian type, they form a very 
well-defined group, and one very distinctly separated from the Ganoids. 
No doubt the Chondrostean Ganoids are nearly as far removed from the Teleostei 
as from the Dipnoi, but the links uniting these Ganoids with the Teleostei have been 
so fully preserved in the existing fauna of the globe, that the two groups almost run 
# As instances of this we may cite (1) the spiral valve; (2) the freqnent presence of a spiracle; (3) 
the frequent presence of a communication between the pericardium and the body-cavity; (4) the hetero- 
cercal tail. 
f Yide F. M. B&lfottk, “ On the Development of the Skeleton of the Paired Fins of Elasmobranchs,” 
Proc. Zool. Soc., 1881. 
