- 1893.] Zoology. 391 
The genera having species in both oceans practically all belong to 
one of three classes: 1st, Tropical genera; 2d, Arctic genera; 8d, 
Pelagic and other genera (Ann. N. Y. Acad. Sci., vi, 1892). 
On the Mechanical Genesis of the Scales of Fishes.— 
Mr. J. A. Ryder has advanced a theory that the genesis and arrange- 
ment of the scales of fishes is due to the movements of the fishes in 
swimming. The scales originatein a continuous subepidermal matrix 
which may be regarded as a basement membrane. Such a matrix is 
found to actually exist in some forms at an early stage, just beneath 
the epidermis; in other forms it exists in the larval stages, and in the 
young of many types of fishes it is coextensive with the whole epider- 
mal layer just at the time when the scales are forming. 
In the Clupeoids and Cyprinoids it is found that the myocommata, 
or sheets of connective tissue intervening between successive somites 
are attached firmly to the deeper layers of the skin. This construc- 
tion, together with the peculiar arrangement of the muscle plates at 
the time the scales begin to form, conditions the further growth of the. 
scale matrix. The ordinary movements of the fish in swimming 
throw the whole integument into definitely circumscribed areolæ, of 
which the central portions are in a quiescent state, while the margins 
are wrinkled or folded as a result of the current action of the lateral 
muscles of the body. 
` In each of the circumscribed areolz a scale develops; the continuity 
of its development with its fellows across the margins of the areola is 
prevented by the flexures to which the dermis is there subjected. 
The author shows that the imbrication as well as the arrangement of 
the scales is determined by the action of the segmentally arranged 
muscles of the body. 
Special types of squamation, as those of the eel and the sturgeon, 
confirm the author’s views. In cases where the scales are very fine, 
that is, several oblique rows corresponding to each somite, the principle 
holds good, since the rows still conform to the lines of tetision of the 
linear attachment of the myocommata to the integuments. 
From the hypothesis and the evidence presented, Mr. Ryder draws 
the following important conclusion : 
1. “ The scales of fishes bear a segmental relation to the remaining 
hard and soft parts, and are either repeated consecutively and in 
oblique rows corresponding to the number of segments, or they may 
be repeated in rows as multiples of the somites, or segmental reduction 
may oceur, which may affect the arrangement of the scales so as to 
