III. COTTOIDS OF NORTH AMERICA. 25 
region. The enlarged one (1, 2) is the sixth; whilst number 3 is the sccond. 
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 hzemapophyses ; the second 
and third (5,6) have a complete hxemal arch on the base of which the pelvic 
appendages (4) are inserted. The hemal arches of the caudal vertebra, as well 
as the hemal spines (/ s) are but little developed. The hemal spines are soldered 
with the arches. The neural and hemal arches and spines are flattened near the 
tail. The caudal plates (7’ h’) of the last vertebra (12) are subtriangular and 
elongated ; the uppermost lined with a few rudimentary rays. 
The ribs or pleurapophyses (p/) are slender, flexible, elongated, and curved, 
needle-shaped spines. The anterior pair is fastened to the scapular arch. 
The interneural and interhemal spines are uniform, elongated and slender, 
differing scarcely from the ribs themselves. 
§ 6. TricLorsis THompsoni1 comPARED wiTH CorTUS 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 7. Thompsonii resembles more that of A. vir- 
gimianus 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 7. T’hompsonii 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 7. Thompsonii 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 7. 
Thompsonii 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 
