28 MONOGRAPH OF THE FRESH WATER III. 



The difference between the encephala of A. nir!<il>i/!* (V\%. 40 48) and A. 

 liniaiim (Fig. 33 35) nre so obvious tbat tbey need scarcely to be pointed out. 

 As usual, the cerebellum varies but little. The mesenceplmlon is elliptic in the 

 former and ovoid in the latter. The prosencephalon of A. niri<il,ilix is uniform 

 above, whilst in a A. vinjinnnius it assumes a character similar to that of C. r /-<"..//.. 

 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 thepyriform rhinenccphalic lobes are seen completely ex pose; 1 

 and free. The same lobes are in close contact and slightly overlapped in A. 

 vari<i/>i/ix. The hypophysis in A. vinjiui'imut, is exceedingly small compared to 

 its size in A. rariabilis, where it is greater than the hypoaria, on which it en- 

 croaches considerably. It is regularly oblong shaped. 



The encephalon of T. 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 subcircuhir, 

 the latter resembling most those in C. grarill*. 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. 

 vwcosua and graciHs, behind and below the hypoaria. In T. Thompsonii the medial 

 lobe is larger than the hypoaria themselves, which are partly covered by it. The 

 haematosac is seen above, overlying the anterior portion of the medial lobe. 

 Immediately in advance of the haematosac, 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. r<irinlii/ix 

 (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. TlIK MfSCULAR SYSTEM (IF CoTTfS Yi.-msi B. 



1'IVTEIII. Fig. 14. 



It has been shown by recent labors that the llc-liy 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 /"/' /v// ///>//), but is actually composed of a series of vertical 

 muscles, the vertical flakes or segments, uliicli correspond generally in number to 

 that of the vertebrae. These flakes or segments (inyocomma), extending from 



