MADE AT THE PLYMOUTH LABORATORY. 257 
form is distinguished from Limanda by the presence of an accessory 
branch of the lateral line, which starts from the anterior portion of 
that line, extending in all members of the family above the eyes, and 
runs backward along the base of the dorsal fin. It is a curious fact 
that this variation is known as a constant character only in the 
Pacific, and that there it occurs in a large number of species : of 
the thirteen species of this sub-family in that ocean distinguished by 
Jordan and Goss it occurs in eight ; and it also occurs in one genus, 
also in the Pacific, in the quite distinct sub-family Hippoglossine. 
A fact of this kind cannot be explained simply as an adaptation ; it 
cannot be supposed that there is some common peculiarity in the 
habits and surroundings of all these species which renders this 
particular extension of the lateral line useful. 
The plaice (Pleuronectes platessa) is distinguished from P. limanda 
by wanting the arch to the lateral line, having cycloid reduced 
scales and tubercles on the post-ocular ridge. The flounder has 
differentiated scales, most of them being reduced and cycloid as in 
the plaice, but those along the bases of the longitudinal fins, along 
the lateral line, and on the head being enlarged to form rough 
spiny tubercles. Pseudo-pleuronectes americanus, the representative 
of the plaice on the east coast of America, approaches the dab in 
having imbricated ctenoid scales. In Liopsetta Putnami the spinu- 
lation of the scales is a sexual character, the scales in the male 
being rough and strongly ctenoid, in the female smooth and almost 
completely cycloid. This species ranges from Cape Cod to Labrador. 
Mr. Holt, in the last number of the Journal, has described specimens 
of the plaice from the Baltic in which ciliation or spinulation of the 
scales, although varying in different individuals, was distinctly a 
sexual character more strongly developed in the males. Mdébius, in 
his Fishes of the Baltic,* mentions these ciliated plaice, and observes 
that they form a transition to unusually smooth specimens of the 
flounder. The smooth flounders, although occurring on the south 
coast of England, are stated to be commonest in the Mediterranean, 
where they seem to occur exclusively. It appears, therefore, that 
there is a northern rough variety of the plaice and a smooth variety 
of the flounder in the south. 
We have at present no evidence that these differences are adap- 
tational, nor can we trace them to preceding or determining causes. 
But, on the other hand, we must admit adaptation in many of the 
characters of the sub-family. or instance, the small size and asym- 
metrical shape of the mouth correspond to the general habit of these 
fish of feeding on invertebrate slow-moving creatures on the sea 
bottom. The fish seize their prey from above with the lower side of the 
* TV'te Bericht der Comm, zur Unters. der Deutschen Meere, 
