INVERTEBRATE PALAEONTOLOGY. 107 



Genus YOLDIA, Moller. 



Synoii.— ToWo, H. P. C. Moller (1842), Kroyer>8 Nat. Tidssk., IV, 91; and (1844), Ind. Moll. Grcenl., 



18, of. Zeitsohr. f. Moll., 12.— H. & A. Ad. (1858), Gen. Recent Moll. II, 584.— Chenu 



(1862), II, 180 ; and of many others. 

 Moldia, Gray (misprint), 1847. 

 Portlandia, Morch (18 ).— H. ami A. Adams (1858), Genera Recent Moll., II, 652; and others (not 



Chenu (1862), Man. Conch., II, 180). 

 Leda and Nucula (species) of some authors ; but not L. of Schuin., or N. of Lam., as properly 



restricted. 



Etym. — Dedicated to the Countess Yoldi. 

 Examp. — Xucula Umatilla, Say. 



Shell transversely subovate, subelliptic, or trapezoidal, thin, narrowed 

 and compressed, or wider and truncated posteriorly, more or less inequi- 

 lateral ; surface smooth, or finely striated, and covered by a polished epider- 

 mis; free margins smooth within; inner layer slightly pearly; denticles of 

 hinge small, and more or less numerous on each side of beaks. Pallial line 

 distinctly sinuous. 



Two sections of this genus are admitted by some as subgenerically 

 distinct ; while others do not seem to regard them as differing sufficiently to 

 be separated in any way. These sections may be distinguished as follows : 



1. yoldia, Moller (typical.) 



Shell usually depressed or narrow subovate, with posterior side 

 often narrowed, and a little gaping.— Nucula limatula, Say. 



2. PORTLANDIA, Morcll. 



Shell proportionally broader (higher), vertically or obliquely 

 truncated and closed behind. — Nucula glacialis, Gray; and N. thraci- 

 formis,Storer. 



These distinctions, however, do not seem to be of much importance ; 

 there being intermediate forms that might with about as much propriety be 

 referred to one as the other of the sections. Both H. and A. Adams and Chenu 

 recognize Portlandia, however, as a distinct subgenus ; but by some strange 

 mistake, the latter author cites two fossil species (Nucula Hcesendonkii, Nyst, 

 from the Tertiary, and N pectinata, Sowerby, from the Cretaceous) as 

 examples; both of which seem to be true nuculas, and certainly cannot be 

 properly included either in a subgenus, or otherwise, under Yoldia; while 

 they are quite as distinct from the recent typical species of Portlandia. 



The genera Yoldia and Nuculana sometimes so nearly resemble each 



