60 PALAEONTOLOGY OF THE UPPER MISSOURI. 



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



1. BTiiciilaiiinae. 



Shell pearly within ; ligament internal. 

 Including Nuuulana and Yoldia, 



3. Malletinae. 



Shell with ligament external ; valves sometimes slightly pearly within. 

 Includes Malletia and Neilo. 



Subfamily NUCULANINiE. 

 Genus YOLDIA, Moller. 



Yoldia, H. P. C. Moller, Krbyer's Nat. Tid. 1842, IV, 91 ; Ind. Moll. Sroen. 18 ; Cf. Zeitschr. f. Mai. 1844, 12. 

 Synan. — Maldia, Gray (misprint), 1847. Leda and Nucula (sp.), of various authors. 

 Etym. — Dedicated to the Countess Yoldi. 

 Examp. — Nurtila limotnla, Say. 



Shell ovate or subcUiptical, 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 laminte slightly 

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

 cartilage occupying a pit under the beaks, PalUal line distinctly sinuous. 



The genus Yoldia is closely allied to Nucidana, but may be distiaguished by its 

 deeper pallial 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 m the paUial line, but by its less distinctly 

 nacreous, and diiferently formed shell. The animals m these two genera are also 

 different. 



We are not sure this genus dates back to the Palteozoic 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 {= 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 arc chiefly found in northern and antarctic seas, and occur on the coast of 

 Greenland, Kamtschatka, Massachusetts, &c. 



Yoldia? siibscitiila. 



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

 Leda subscitula, Meek & Haydes, Trans. Albany Inst. IV, March 2d, 1858. 



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

 half more compressed and aubrostrate, 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 tbem. Beaks moderately prominent, and nearly central. Hinge straight or sloping slightly frnra 

 the beaks, near wliich a few fine crenulations are visible in the cast. 



Length, 0.50 inch ; height, 0.26 inch; breadth or convexity, about 0.17 inch. 



