MADE AT THE PLYMOUTH LABORATORY. 253 
its colour to that of the rock surface is well concealed from obser- 
vation, and in this way its habit is an advantage to it, just as the 
habit of covering itself with sand is of advantage to a sole. 
But according to the present state of our knowledge the only 
generic character which is necessary to the peculiar habit is the 
great development of the posterior muscles and fins, which is the 
chief factor in the characteristic shape of the body. It is possible 
that the roughness of the upper surface, due to the spines of the 
scales, adds to the concealment of the fish by aiding in the resem- 
blance to a rock surface. But no use has yet been found for the 
subcaudal prolongations of the fins or the perforation of the gill- 
septum. 
With regard to the specific characters, nothing is known of 
peculiarities in mode of life which would give an importance in the 
struggle for existence to the concrescence of the pelvic fins with the 
ventral in punctatus, to the absence of this character and the 
elongation of the first dorsal ray in wnimaculatus, or to the absence 
of both characters in norvegicus. No use is known, in fact, for any 
of the other specific characters, of which a brief review may be here 
given. The characters tend to form a series. Thus, in size, 
norvegicus is smallest, wrimaculatus larger, and punctatus largest, 
the last reaching a length of 84 inches. The subcaudal fin-flaps 
are least developed in norvegicus ; most in punctatus ; each has four 
rays in norvegicus and wnimaculatus, six im punetatus. The 
shortening and spinulation of the scales are greatest in punctatus, 
least in norvegicus. In punctatus there are teeth on the vomer, in 
unimaculatus none, in norvegicus they are very small. 
According to the studies of Jordan and Goss, the flat fishes form 
three well-marked sub-families, including the most numerous and 
important forms, and sundry other sub-families of less importance. 
These three are those whose familiar British representatives are the 
turbot, the plaice, and the sole; they may be called the Rhombine, 
Pleuronectinz, and Soleine. I do not follow the American authors 
in their application of the names. The Rhombine have the 
following principal distinguishing characteristics. 
The body is sinistral. The mouth is symmetrical, the dentition 
nearly equally developed on both sides, and the teeth acute. 
Pectorals and pelvics usually well developed, and the pelvics asym- 
metrical, that of the left or eyed side inserted on the extreme edge 
of the abdomen, its rays more or less wide apart; that of the right 
side inserted on the right side. Caudal fin rounded or subtruncate. 
Vertebree in moderate or small number. Species chiefly tropical or 
subtropical. 
It is to this division that Zeugopterus belongs. The nearest 
