allied fish, assigned to a different genus, under the name of Sternoptyx diaphana. The deformation illustrated by this case of Argyropelecus is precisely analogous to the simplest and commonest kind of deformation to which fossils are subject (as we have seen on p. 553) as the result of shearing-stresses in the solid rock.
Fig. 375. Scarus sp. Fig. 376. Pomacanthus.
Fig. 375 is an outline diagram of a typical Scaroid fish. Let us deform its rectilinear co-ordinates into a system of (apôÙproxôÙiôÙmateôÙly) coaxial circles, as in Fig. 376, and then filling into the new system, space by space and point by point, our former diagram of Scarus, we obtain a very good outline of an allied fish, belonging to a neighbouring family, of the genus Pomacanthus. This case is all the more interesting, because upon the body of our Pomacanthus there are striking colour bands, which correspond in direction very closely to the lines of our new curved ordinates. In like