160 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



now and then of some of the mutations necessary 

 to complete the series. A very clear example of 

 these discontinuous series is offered by the evolution 

 of the Gryphsea, a genus akin to our oysters, but 

 distinguished by a deep left valve with a top more 

 or less incurved, and a flat and opercular right 

 valve. The earliest species is the Gryphcea arcuata, 

 which forms immense beds in the lower Lias, and 

 presents in a high degree the characteristics of the 

 genus, viz. : a narrow and deep shape of the large 

 valve, the top of which is very thick, and sharply bent 

 over on to the small valve. As early as the higher 

 part of the Lias a first mutation is met with which 

 has received the name of Gryphcea obliqua. The 

 shell here is less deep, wider, and the top smaller 

 and less incurved. If we go to the middle Lias, 

 another form is found : the Gryphcea cymbium, a 

 little larger in size, and the shell wider, much deeper, 

 with a top thin and slightly bent back. Then the 

 series momentarily disappears in the higher Lias, 

 the Bajocian and the Bathonian, to reappear in 

 the Callovo-Oxfordian marls under a form slightly 

 modified, larger and more spread out in width, 

 which has rightly received the name of Gryphcea 

 dilatata. The branch ceases to show itself in the 

 last stages of the Jurassic, and in the whole of the 

 lower Cretacean, and when it reappears in the 

 higher Chalk with the Gryphcea proboscidea and 

 vesicularis, the characteristics are strongly modified : 

 the general form of the shell is much greater in 

 width and almost circular, the top is blunt and but 

 little bent over as if merged into the rest of the 

 shell. Its general appearance so slightly resembles 



