of the various {748} hydroids which we inscribe therein (Fig. 372). The comparative smoothness or denticulation of the margin of the calycle, and the number of its denticles, constitutes an independent variation, and
requires separate description; we have already seen (p. 236) that this denticulation is in all probability due to a particular physical cause.
Among the fishes we discover a great variety of deformations, some of them of a very simple kind, while others are more striking and more unexpected. A comparatively simple case, involving a simple shear,
Fig. 373. Argyropelecus Olfersi. Fig. 374. Sternoptyx diaphana.
is illustrated by Figs. 373 and 374. Fig. 373 represents, within Cartesian co-ordinates, a certain little oceanic fish known as Argyropelecus Olfersi. Fig. 374 represents precisely the same outline, transferred to a system of oblique co-ordinates whose {749} axes are inclined at an angle of 70ô¯; but this is now (as far as can be seen on the scale of the drawing) a very good figure of an