248 TOWARDS THE HUMAN FORM 



climate, as we can tell by the presence at certain points of 

 lateritic minerals, which can be formed only under the 

 action of intense solar radiation. Douville has discovered 

 Orbitolites — large circular Foraminifera — which only inhabit 

 warm seas abounding in lime, wherever there are reefs of 

 Rudistse. They presage the imminent arrival of the 

 Nummulites which will play so great a part in the seas of 

 the Eogene period. 



We cannot help being struck by the modifications produced 

 in the habits of animals during Jurassic times. In the 

 preceding period almost all the Gasteropods had shells with 

 entire openings ; these organisms lived exclusively upon 

 vegetable food, and the pulmonate Molluscs which also have 

 shells with entire openings, and which had invaded both 

 land and fresh water, are likewise almost exclusively 

 vegetarian. The carnivorous Gasteropods, the opening of 

 whose shell is either notched or drawn out into a tube, did 

 not make their appearance until the Secondary Period. This 

 correspondence between the diet and the shell aperture is 

 not due simply to chance. The carnivorous Molluscs are led 

 to their prey by the sense of smell. As soon as a dead body 

 falls into the water it is surrounded on all sides by Nassse. Now 

 the olfactory organ of the Gasteropods, the osphradium or false 

 gill, is situated in the branchial cavity near the true gill. A 

 tube or siphon, formed by a prolongation of the fleshy top of 

 the branchial chamber, conducts the water into this chamber, 

 and on to both the osphradium and the branchiae, whose 

 functions are thus regulated. This siphon fits either into the 

 notch or the canal of the opening of the shell, and we can 

 see that its gradual elongation was due to the efforts of the 

 mollusc to induct the maximum quantity of odoriferous 

 effluvia within its reach. 



Was it the great number of these preying molluscs that drove 

 certain of the Lamellibranchs to adopt the life of the recluse ? 

 Their senses are so rudimentary that they can hardly be credited 

 with sufficient intelligence to carry out a deliberate intention. 

 It is, however, undeniable that during the Primary Period those 

 Lamellibranchs living on the surface of the ground and 

 crawling by means of a foot, somewhat similar to that of the 

 Gasteropods, or suspended by a byssus, were in the majority. 

 Whereas, in the course of the Secondary those Lamelli- 



