GO PALAEONTOLOGY OF THE UPPER MISSOURI. 



This group has been divided by Conchologists into two subfamilies, as follows : 



1. Niiculaiiinae. 



Shell pearly within ; ligament internal. 

 Including Nuculana and Yoldia. 



2. Malletinae. 



Shell with ligament external ; valves sometimes slightly pearly within. 

 Includes Malletia and Ncilo. 



SUBFAMILY NUCULANIM. 

 Genus YOLDIA, MOLLER. 



Yoldia, H. P. C. MOLLER, Kroyer's Nat. Tid. 1842, IV, 91 ; Ind. Moll. Green. 18 ; Cf. Zeitschr. f. Mai. 1844, 12. 

 SynM. Maldia, GRAY (misprint), 1847. Leda and Nucula (sp.), of various authors. 

 Etym. Dedicated to the Countess Yoldi. 

 Examp. Nucula limatula, SAY. 



Shell ovate or subelliptical, subequilateral, more or less compressed ; posterior 

 side narrower than the other. Surface smooth, striate or obliquely sculptured, and 

 covered with a polished epidermis. Margins smooth within ; inner laminae slightly 

 pearly. Hinge plaits small, and more or less numerous on each side of the beaks ; 

 cartilage occupying a pit under the beaks. Pallial line distinctly sinuous. 



I 



The genus Yoldia is closely allied to Nuculana, but may be distinguished by its 

 deeper pallia! sinus, and usually less prominent beaks. From Nucula, which it 

 resembles in the crenulated character of the hinge, it will be readily distinguished, 

 not only by the presence of a sinus in the pallial line, but by its less distinctly 

 nacreous, and differently formed shell. The animals in these two genera are also 

 different. 



We are not sure this genus dates back to the Palaeozoic epoch, though some of 

 the Carboniferous and Permian species present exactly 'the form and external 

 appearance of true Yoldias. Some of the Triassic and Jurassic species usually re- 

 ferred to the closely allied genus Nuculana (j= Leda), probably also belong to this 

 genus. Several of the Cretaceous species, figured by D'Orbigny in the Palaeon- 

 tology of France, under the names Nucula and Leda, seem to be typical Yoldias. 



The genus Yoldia was represented by a few species during the Tertiary epoch, 

 and probably attains its greatest development in the existing seas. The recent 

 species are chiefly found in northern and antarctic seas, and occur on the coast of 

 Greenland, Kamtschatka, Massachusetts, &c. 



Yoldia? siibscitula. 



(PLATE II, Fig. 4, a, b.) 

 Leda subscitula, MEEK & HAYDEN, Trans. Albany Inst. IV, March 2d, 1858. 



Shell of medium size, rather narrow subovate, moderately convex in the central and nmbonal regions. Posterior 

 half more compressed and subrostrate, very narrowly rounded at the extremity ; anterior extremity less narrowly 

 rounded. Base forming a broad semi-ovate curve, the most convex part of which is slightly in advance of the 

 middle ; dorsal outline convex, and declining a little in front of the beaks, somewhat concave and nearly hori- 

 zontal behind them. Beaks moderately prominent, and nearly central. Hinge straight or sloping slightly from 

 the beaks, near which a few fine crennlations are visible in the cast. 



Length, 0.60 inch ; height, 0.26 inch ; breadth or convexity, about 0.17 inch. 



