878 



D'ARCY WENTWORTH THOMPSON ON 



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 com- 

 paratively simple case, involving a simple shear, is illustrated by figs. 29 and 30. 

 Fig. 29 represents, within Cartesian co-ordinates, a certain little oceanic fish known as 

 Argyropelecus Olfersi. Fig. 30 represents precisely the same outline, transferred to 



Fig. 28.— a, Cladocarpus crenatus, F. ; b, Aglaophenia pluma, L. ; c, A. rhyncocarpa, A. 

 d, A. cornuta, K. ; e, A. ramulosa, K. 



a system of oblique co-ordinates whose axes are inclined at an angle of 70° ; but this 

 is now (as far as can be seen on the scale of the drawing) a very good figure of an 

 allied fish, assigned to a different genus, under the name of Sternoptyx diaphana. 

 Fig. 31 is an outline diagram of a typical Scaroid fish. Let us deform its 



l''ic. 29. — Argyropelecus Olfersi. 



Fir,. 30. — Sternoptyx diaphana. 



rectilinear co-ordinates into a system of (approximately) coaxial circles, as in fig. 32, 

 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 Pornacanthus. This case is all the more interesl 

 ing. because upon the body of our Pornacanthvs there are striking colour bands, which 

 correspond in direction very closely to the lines of our new curved ordinates. In 

 like manner, the still more bizarre outlines of other fishes of the same family of 



