28 MONOGRAPH OF THE FRESH WATER IIL. 
The difference between the encephala of A. variabilis (Fig. 46—48) and A. 
virginianus (Fig. 33—35) are so obvious that they need scarcely to be pointed out. 
As usual, the cerebellum varies but little. The mesencephalon is elliptic in the 
former and ovoid in the latter. The prosencephalon of A. variabilis is uniform 
above, whilst in a A. virginianus it assumes a character similar to that of C. viscosus 
with this difference, that the posterior and inner hilly protuberance acquires a 
much greater development, and seems to absorb the two others. In advance 
of the prosencephala the pyriform rhinencephalic lobes are seen completely exposed 
and free. The same lobes are in close contact and slightly overlapped in A. 
variabilis. The hypophysis in A. virginianus, is exceedingly small compared to 
its size in A. variabilis, where it is greater than the hypoaria, on which it en- 
croaches considerably. It is regularly oblong shaped. 
The encephalon of 7. Thompsonii may readily be distinguished from that of either 
Cotti or Acanthocotti. The epencephalon exhibits a more prominent swelling on the 
sides of the medulla oblongata. The cerebellum and mesencephala are subcircular, 
the latter resembling most those in C. gracilis. The prosencephalon is proportionally 
much more developed than either in Cotti or Acanthocotti, and appears also more 
separated from the mesencephalon. At the inner and posterior edge, a circular 
swelling exists, looking like a smaller pair of lobes superadded on that region. 
The rhinencephalon is so much overlapped that it is only apparent in the view from 
underneath (Fig. 37). The hypophysis is of medium size, situated in advance of 
the mesencephalic lobes. The hypoaria seem rather secondary in importance; 
they are absorbed by the development of a medial lobe faintly indicated in C. 
viscosus and gracilis, behind and below the hypoaria. In 7. Thompsonii the medial 
lobe is larger than the hypoaria themselves, which are partly covered by it. The 
heematosac is seen above, overlying the anterior portion of the medial lobe. 
Immediately in advance of the hematosac, on the hypoarian floor, a small spherical 
swelling is slightly indicated, and surrounded in front by a ridge delineating the 
anterior limit of this floor. 
Without having contemplated the comparative study of the cephalic nerves in 
this memoir, we have allowed them to be represented in the case of A. variabilis 
(Figs. 46—48), in which we had a fair chance to observe them. This may be 
interesting to anatomists, who might wish to compare their development and 
distribution with that of other groups. 
§ 8. THE MUSCULAR SYSTEM OF COTTUS VISCOSUS. 
Puate III. Fig. 1—4. 
It has been shown by recent labors that the fleshy mass which extends from 
head to tail, all along the sides of the body of fishes, does not constitute a single 
muscle (the so-called dateral muscle), but is actually composed of a series of vertical 
muscles, the vertical flakes or segments, which correspond generally in number to 
that of the vertebrae. These flakes or segments (myocomma), extending from 
