30 EVOLUTION OF FINS 
which ‘apparently was imbedded in the integument; its 
gouge-like form (Figs. 37 and 38) permitted it to be firmly 
apposed to its anterior and posterior neighbours. Each — 
median enamelled cusp represents apparently the sum of 
the shagreen papillz, occurring in the median-dorsal region 
of each metamere, its gouge-like underlying portion the 
metameral calcification of the bases of the denticles. 
What has been the mode of origin of the primitive 
derm cusps is a puzzling question. It is significant, per- 
haps, that they occur in primitive forms (sharks) in con- 
nection with the sense organs of the lateral line (p. 50), 
and that they are in this region retained in a number 
of archaic forms (Polypterus, p. 148, Callichthys, p. 172), 
which have in all other body parts evolved protective derm 
plates.* It is certain that for the sensory groove of the 
lateral line, no more simple, protective devices could have 
arisen than conical elevations of skin. Arising in this 
region, they may have extended their protective functions 
over the entire body surface. 
3. THE EVOLUTION OF FINS 
Fins are the organs of progression adapted to the 
needs of aquatic living. A fish, balanced in its living 
medium, acquires, as has been seen, a boat-like form, 
enabling it to pierce the water in the least resisting 
manner. Its appendages, when they come to arise, must 
reasonably be looked to to fulfil the mechanical condi- 
tions of aquatic motion in order to propel to the best 
advantage the lightly balanced and boat-shaped mass. 
Fins might thus be expected to arise as keel-like struct- 
*In the sensory canals of the head of Chimera, the presence of scattered 
bony plates, protective in function, v. p. 114, would suggest the concentration 
of the marginal cusp elements for more perfect protection. 
