266 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



close to the surface. In progressive growth, as the 

 animal burrows deeper, the siphon elongates, until it 

 attains a length many times the total length of the 

 valves. The ontogeny of the individual and the pale- 

 ontology of the family both show that Mya came from 

 a form with a very abbreviated siphon, and it seems 

 evident that the long siphon of this genus was brought 

 about by the effort to reach the surface, induced by the 

 habit of deep burial. 



" The tendency to equalize the form of growth in a 

 horizontal plane in relation to the force of gravity act- 

 ing in a perpendicular plane, or the geomalic tendency 

 of Professor Hyatt,^ is seen markedly in pelecypods. 

 In forms which crawl on the free borders of the valves 

 the right and left growth in relation to the perpendic- 

 ular is obvious, and agrees with the right and left sides 

 of the animal. In Pecten the animal at rest lies on 

 the right valve, and swims or flies with the right valve 

 lowermost. Here equalization to the right and left of the 

 perpendicular line passing through the center of grav- 

 ity is very marked (especially in the Vola division of 

 the group) ; but the induced right and left aspect cor- 

 responds to the dorsal and ventral sides of the animal, 

 — not the right and left sides, as in the former case. 

 Lima, a near ally of Pecten, swims with the edges of 

 the valves perpendicular. In this case the geomalic 

 growth corresponds to the right and left sides of the 

 animal. 



"The oyster has a deep or spoon-shaped attached 

 valve and a flat or flatter free valve. This form, or a 

 modification of it, we find to be characteristic of all 



1 "Transformations of Planorbis at Steinheim, with Remarks on the Ef- 

 fects of Gravity Upon the Forms of Shells and Animals." Proceeds. A. A. A. 

 S., Vol. XXIX., 1880. 



