III. COTTOIDS OF NORTH AMERICA. 25 



region. The enlarged one (1, 2) is the sixth; whilst number 3 is the second. 

 The numbers 4, 5 and 6, are pelvic vertebra; and 7 to 12 belong to the caudal 

 region. 



The first pelvic vertebra (4) shows the first traces of hsemapophyses ; the second 

 and third (5, 6) have a complete haemal arch on the base of which the pelvic 

 appendages (4) are inserted. The hajmal arches of the caudal vertebras, as well 

 as the hsemal spines (Ji s) are but little developed. The hsemal spines are soldered 

 Avith the arches. The neural and hajmal arches and spines are flattened near the 

 tail. The caudal plates (»' A') of the last vertebra (12) are subtriangular and 

 elongated ; the uppermost lined with a few rudimentary rays. 



The ribs or pleurapophyses {pi) are slender, flexible, elongated, and curved, 

 needle-shaped spines. The anterior pair is fastened to the scapular arch. 



The interneural and interhaemal spines are uniform, elongated and slender, 

 differing scarcely from the ribs themselves. 



§ 6. Triglopsis Thompsonii compared with Cottus viscosus, and with Acantho- 



cottus virginianus. 



The plan of structure of the bony frame of the three genera of Cottoids which 

 we have had an opportunity of examining, is identical in its main features. 



The general form of the skull of T. Thompsonii resembles more that of A. vir- 

 ginianus than that of C. viscosus or any other species of Cottus. The chief differ- 

 ence consists in the absence of spines, and in this respect T. Thompsonii comes 

 nearer to C viscosus than to A. virginianus. The turbinals in the latter are strong 

 and spiny, whilst in the two others these same bones are weak, thin, scaly and 

 deprived of spines. The cephalic channels of the lateral line are identical in the 

 three genera, but reaches its maximum of development in Triglopsis, whilst it is 

 much less apparent in Cottus and Acanthocottus. The suborbital series is the same 

 in the whole group, only a little modified in the shape of its constitutive pieces 

 and their degree of union. The hyoidean and branchial arches are identical. The 

 opercular apparatus assumes differences by which the genera may be distinguished. 

 In T. TJiompsonii the preopercular is provided with four needle-like spines, short 

 and flexible. C. viscosus has only one well-developed spine, whilst in A. virgini- 

 anus besides the stout spines of the preopercular, we see the opercular, the sub- 

 opercular, the interopercular, and the hypotympanic, provided with similar spines ; 

 the mesotympanic and pretympanic are also more compact, and better defined in 

 the latter species. 



The scapular arch has the same general structure ; but in A. virginianus the 

 angles of the suprascapular and scapular are more acute and spine like; in T. 

 Th&inpsonii the ulna and radius coalesce, thus establishing a separation between 

 these bones and the coracoid. 



The ventral fins in the three genera have the same position under the head and 

 the same connection with the scapular arch. The pubic bones articulate together 

 4 



