76 PALEONTOLOGY 



and the different types of gill give us grades of development 

 only, not lineages. 



The latest proposal for classification (here adopted) is 

 that of Professor Henri Douville, of Paris, according to 

 whom the main lines of divergence of the lamellibranchs 

 were determined by original differences in habits of life. 

 There are three branches active, fixed, and burrowing 

 so that we have here what has been termed by Verte-, 

 brate palaeontologists an adaptative radiation. But the 

 members of any lineage are not rigidly tied down to one 

 groove ; in each lineage there are cases of re-adaptation 

 to one of the other modes of life, re-adaptations which 

 lead sometimes to a close imitation of one type by 

 another (convergence), but which can never obliterate 

 or reverse the effects of ancestral history. 



I. Active Branch. 



I. NUCULACEA. 



Equi valve; isomyarian; internally nacreous; hinge-line 

 primitively taxodont ; without distinct cardinal area ; 

 surface rarely ornamented otherwise than by concentric 

 striae. Ligament external (Cienodonta) or internal (Nttcula, 

 Ledd). Integripalliate (except Leda and Yoldia). Ordo- 

 vician to Recent. 



Leda (Ord.-Rec., Fig. 23, a) and Yoldia differ from Nucida 

 in having the mantle drawn out into distinct though small 

 siphons. This greater development of the posterior region 

 results in a more central umbo, while there is a slight 

 pallial sinus, and in Leda a tendency to more or less 

 rostration (drawing out of the posterior end) : species 

 which show this in an extreme degree are sometimes 

 separated under the name Dacvyomya. Nucula and Leda 



