748 



THE THEORY OF TRANSFORMATIONS 



[CH. 



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 



Fig. .372. a, Cladocarpus crenatus, F. ; b, Aglaophenia pluma, L. ; c, A. 

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



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 



Fig. 373. Argyropelecus Olfersi. 



Fig. 374. Sternoptyx diaphana. 



simple shear, is illustrated by Figs. 373 and 374. Fig. 373 repre- 

 sents, mthin Cartesian co-ordinates, a certain little oceanic fish 

 known as Argyropelecus Olfersi. Fig. 474 represents precisely the 

 same outline, transferred to a system of oblique co-ordinates whose 



