26 MONOGRAPH OF THE FRESH WATER III. 



into a triangular piece whicli 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 vertebrse varies in the different genera according to the 

 length of their body. That number even varies amongst the different species of 

 the same genus. But the centrum of the vertebrae 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 T. 

 Thompsonii, G. viscosus, and A. virginianus. Their absolute number may vary 

 according to similar variations in the number of abdominal vertebrae. As far as 

 our investigations go, we found constantly three pelvic vertebrae, calling by that 

 name such vertebrae 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 G. viscosus and T. TJiomp- 

 sonii, they are represented hanging down, in order to render them more conspicuous. 

 The caudal vertebrae, again, are liable to some variation in number. 



The interneural, and interhaemal spines, as well as the dermo-neural and dermo- 

 haemal ones, present the greatest similarity in the whole group. 



§ 7. The encephala of Cottus viscosus, C. gracilis, C. gobioides, Triglopsis 

 Thompsonii, Acanthocottus virginianus, and A. variabilis, compared. 



Plate 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 



