20 ■ MONOGRAPH OP THE FRESH WATER III. 



The front of the vomer is different both from G. viscosus and G. Franldinii. 

 The acoustic caj^sules rise also more above the surface of the sphenoid. 



Of G. Ricliardsonii (Fig. 18 — 21) we had only a small specimen to carry out 

 our osteological comparisons. The skull resembles more that of G. Franldinii 

 than any other; but the upper view (Fig. 18) shows the difference between the two 

 species. In G. Ricliardsonii the form is subcircular, the width near the orbits being 

 equal to that on the occipital region. 



V Thus we see that species which zoologically differ from each other most, such as 

 G. FranTclinii from G. Ricliardsonii, and C. viscosus from G. Wilsonii, are those 

 in which the skull presents the least difference. 



§ 4. Comparative Osteology op Cottus viscosus and Acanthocottus virginiakus. 



Plate III. Fig. 5—9 and 26—32. 



Nothing is more alike than the general plan of arrangement of the bony frame 

 in G. viscosus and A. virginianiis. The differences must be sought for in the special 

 structure of the regions, and in the preponderance of some of the bones, especially 

 in those of the head. 



In comparing the skull of Gotlus viscosus (Fig. 6 — 9) with that of Acantliocoilus 

 virginianus (Fig. 26 — 29), the most striking difference is found to consist in the 

 fronto-orbital arcade, which is much broader in the latter (Fig. 26) than in the former 

 (Fig. 6). The snout is likewise more developed in Acanthocottus, the nasals 

 occupying the whole space between the external projection of the prefrontals and 

 the vomer. The turbinals in Acanthocottus are ossified, quadrifid, intimately con- 

 nected with the prefrontals and nasals, and surmounted with a stout spine. The 

 nasal spines themselves are partly concealed by the turbinals. The orbital rim 

 rises above the middle smooth region of the arcade. A small spine directed back- 

 wards and outwards exists on the postfrontal, whence a ridge extends along the 

 upper surface of the skull, to the supraoccipital, where a similar spine is seen. 

 Thus the upper surface of the cranial box of Acanthocottus differs from that of 

 Cottus by the presence of spines and ridges. The frontal region, however-, is flat 

 and smooth. The skull in Acanthocottus is proportionally more elongated; the 

 orbito-frontal arcade and the snout together, much longer than the cranial box. The 

 latter is also proportionally narrower near the orbits than on the occipital region. 

 In height, the proportions do not differ much. 



The vomer in Acanthocottus is provided along the medial line of its upper sur- 

 face with a sharp carina extending to the whole length of that bone. Anteriorly 

 there exists a pair of nearly vertical and approximated processes, behind which, and 

 more apart, another pair is seen obliquely inclined backwards and outwards. 



On the sides of the head there is the same number of bones, occupying the same 

 relative position. As to their shape or form, some vary but very slightly, whilst 

 others assume a quite different character. Thus the preopercular is provided with 

 a very much elongated spine, stouter than the body of the bone itself. Immedi- 

 ately underneath, there is a second small but stout spine; both are directed 



