III. COTTOIDS OF NORTH AMERICA. 19 



more compressed than the abdominal ones; this is owing principally to the more 

 erect position of the neural arch, and to the presence of hnemapophyses, build- 

 ing up a haemal arch terminated by haemal spines (h s), also intimately soldered 

 together. The anterior haemal arches are very much bent backwards, in the same 

 manner as the anterior neural arches, but they soon become less so. The haemal 

 canal itself, likewise diminishes backwards. The neural and haemal spines of the 

 last vertebra but one (y) are flattened and expanded. The terminal caudal vertebra 

 is reduced to a very small centrum, which seems to be absorbed by caudal plates 

 (ri 7i'), on the edge of which the rays of the caudal fin (d ri) are inserted. The 

 upper edge of ri, and the lower of li' exhibit a rudimentary spine, no doubt the 

 first indication of the rudimentary rays of each caudal lobe. 



The ribs themselves or pleurapophyses (pT) are needle-like, a little curved, and 

 diminish gradually in length backwards. 



The interneural spines (i n) are variable in form, stouter under the first dorsal fin. 



The interhaemal spines (i h) are more uniform. 



The dermo-neural (d n) and dermo-haemal (d h) spines, are all very much alike, 

 the only difference consisting in the spiny or bony nature of those belonging to 

 the first dorsal fin. 



3. COMPARISONS BETWEEN THE SKULLS OF DIFFERENT COTTI. 



PLATE III. Fig. 621. 



The skull of C. viscosus (Fig. 6 9) as described above (p. 14) must be recalled 

 to mind in order to understand these comparisons. 



The chief difference between C. viscosus and C. FranJdinii (Fig. 10 13) consists 

 in the fronto-orbital arcade which forms nearly a straight line along the declivity of 

 the snout (Fig. 12). Besides, this region is a little shorter than the cranial box 

 itself. The upper surface is likewise depressed, and the depression extends back- 

 wards on the occiput, so that the latter region appears more convex in Coitus 

 FranJdinii (Fig. 13). The cephalic channels of the lateral line are more developed 

 than in C. viscosus ; the upper view (Fig. 10) exhibits very plainly this difference 

 by the reduced smooth surface in the middle, and the larger holes along the circum- 

 ference. The acoustic capsule differs also in both of these species. We might even 

 notice a difference in shape in the front of the vomer, which, in all the species, bears 

 a narrow band of teeth. 



In C. Wilsonii (Fig. 14 17), the fronto-orbital arcade is still lower than in C. 

 Franldinii, and passes gradually to the upper surface of the skull without the 

 slightest rising above it. This region again is of the same length as the cranial 

 box itself. The upper surface of the skull is flat and perfectly even; being a 

 little lower near the orbit than on the occiput; it has a slight sloping forwards 

 (Fig. 16). This character will likewise be found on those species the external 

 appearance of whose snout is rather elongated. Seen from behind (Fig. 17) the 

 upper part seems more convex than it is in reality; this apparent convexity being 

 given to it by the mastoid crests. 



