XVII] THE COMPARISON OF RELATED FORMS 1061 



The comparative smoothness of 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. 391) that this denticulation is in all probability due to a par- 

 ticular physical cause. 



Among countless other invertebrate animals which we might 

 illustrate, did space and time permit, we should find the bivalve 

 molluscs shewing certain things extremely well. If we start with 

 a more or less oblong shell, such as Anodon or Mya or Psammobia, 

 we can see how easily it may be transformed into a more circular 

 or orbicular, but still closely related form; while on the other hand 

 a simple shear is well-nigh all that is needed to transform the 

 oblong Anodon into the triangular, pointed Mytilus, Avicula or 

 Pinna. Now suppose we draw the shell of Anodon in the usual 

 rectangular coordinates, and deform this network into the corre- 

 sponding oblique coordinates of Mytilus, we may then proceed to 

 draw within the same two nets tEe anatomy of the same two molluscs. 

 Then of the two adductor muscles, coequal in Anodon, one becomes 

 small, the other large, when transferred to the oblique network of 

 Mytilus ; at the same time the foot becomes stunted and the siphonal 

 aperture enlarged. In short, having "transformed" one shell into 

 the other we may perform an identical transformation on their 

 contained anatomy : and so (provided the two are not too distantly 

 related) deduce the bodily structure of the one from our knowledge 

 of the other, to a first but by no means negligible approximation. 



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, is illustrated by Figs. 517 and 518. The one represents; 

 within Cartesian coordinates, a certain little oceanic fish known as 

 Argyropelecus Olfersi. The other represents precisely the same out- 

 line, transferred to a system of oblique coordinates 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. The 

 deformation illustrated by this case of Argyropelecus is precisely 

 analogous to the simplest and commonest kind of deformation to 



