26 MONOGRAPH OF THE FRESH WATER IO 0K 
into a triangular piece which penetrates forwards above the union of the coracoids. 
There exists in all, an external and unjointed ray with three or four jointed ones. 
The number of the vertebre varies in the different genera according to the 
leneth of their body. That number even varies amongst the different species of 
the same genus. But the centrum of the vertebra themselves is very much alike in 
all Cottoids. The axis of each vertebra is provided with a minute hole, thus establish- 
ing a direct communication through the centre of the vertebral column, from the 
occiput to the tail. The shape of the neural canal is subjected to some variation, 
according to the greater or less development of the neural arches and neural spines. 
The ribs, or pleurapophyses are identical in form, structure and position in 7. 
Thompsonii, C. viscosus, and A. virginianus. Their absolute number may vary 
according to similar variations in the number of abdominal vertebra. As far as 
our investigations go, we found constantly three pelvic vertebra, calling by that 
name such vertebre as are provided underneath the ribs with rib-shaped, or styli- 
form bones, of which there are constantly three pairs, bent backwards and kept 
within the muscular wall of the pelvis. In the figures of C. viscosus and T. Thomp- 
sonit, they are represented hanging down, in order to render them more conspicuous. 
The caudal vertebre, again, are liable to some variation in number. 
The interneural, and interhemal spines, as well as the dermo-neural and dermo- 
hemal ones, present the greatest similarity in the whole group. 
§ 7. THe ENCEPHALA OF CorTus viscosus, C. GRACILIS, C. GOBIOIDES, TRIGLOPSIS 
THOMPSONII, ACANTHOCOTTUS VIRGINIANUS, AND A. VARIABILIS, COMPARED. 
Prare III. Fig. 33—48. (Figs. 36—45 are represented twice natural size.) 
In the genus Cottus the brain or encephalon fills up the whole cavity of the 
skull, the upper roof of which is almost in contact with the upper surface of the 
encephalon, which can be seen through the frontal bones, as soon as the skin is 
removed. The layer of cellular fat is consequently very thin, as the space itself is 
exceedingly reduced. 
In the genus Acanthocottus, the cavity of the skull would contain a brain twice 
as large. The space all around, is occupied by cellular fat filling up completely 
the entire cavity. When the upper roof of the skull is cut off the encephalon is 
not exposed unless that fat is removed. 
In the genus Triglopsis, we have an intermediate stage; the cavity of the skull 
is still larger than the bulk of the encephalon and the remaining space filled by a 
similar fat. 
If we were to establish a series, we would not hesitate in placing Acanthocottus 
below, next Triglopsis, and Cottus above. 
But let us now glance at the brains themselves, and see how far they can be 
available in comparative zoology. 
The different regions of the encephalon are closely grouped together. There 
exist no rhinencephalic crura carrying off the rhinencephala or olfactory lobes at 
a certain distance from the prosencephala. The rhinencephalic lobes are brought 
