398 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.52. 



extraordinary mutations, which taken alone would appear to represent 

 perfectly distinct species. In the extreme form of this variety the 

 surface of the disk is perfectly smooth. In another mutation there 

 are a few very coarse concentric ridges near the umbones. In still 

 another the umbonal region is smooth and the ridges appear near the 

 basal margin only. The keels on the rostrum in typical limata are 

 plain, in other specimens there may be successive prominent concen- 

 tric ridges rising into pustules where they intersect the keels. Almost 

 every combination of these characters may be found in a large series 

 of the species. These fluctuations appear to be coimected with the 

 southern habitat, being most violent in specimens collected at La Paz, 

 just inside the Gulf of Cahfoniia. A somewhat similar series of mu- 

 tations has been noticed in one of the species from the Miocene of 

 Vhginia. Cat. No. 211292, U.S.N.M. 



Station (of the typical specimens) 2902, off Santa Rosa Island, 

 Cahfornia, in 50 fathoms. 



YOLDIA OLEACINA. new species. 



SheU small, plump, smooth except for faint incremental lines, bril- 

 Hantly pohshed, of a bright yeUow brown with a shght ohvaceous 

 tint toward the beaks; egg-ovate, rounded and broader in front, 

 attenuated behind; base broadly arcuate, dorsal margin gently 

 curved; beaks low, 6 mm. from the anterior end; no defined lunule 

 or escutcheon; 13 anterior and 11 posterior hinge teeth, the resilifer 

 subumbonal, cup-Hke; the paUial Hne has a slight shallow sinus near 

 the posterior adductor scar. Length, 16; height, 9; diameter, 6 mm. 

 Cat. No. 223407, U.S.N.M. 



Arctic Ocean north of Bering Strait, Captain Healy. 



This briUant species recalls several of the Arctic species, but does 

 not exactly agree with any of them. 



YOLDIA SECUNDA, new species. 



Shell large, thin, inequilateral, inflated, subtiimcate and recurved 

 beliind; color of a light grayish ohve, more or less disposed in zones; 

 tliis shell much resembles Y. thraciaeformis Storer, though it does 

 not attain so gi'eat a size; it differs bj^ the absence of the obhque ele- 

 vated posterior ray from the umbones, in being more attenuated 

 behind, and in general more cyhndi-ically inflated; the valves hardly 

 gape in front, and less behind than in that species; the hinge teeth 

 are more numerous and smaller than in thraciaeformis of the same 

 length. There are 24 anterior and 20 posterior teeth, the resilifer is 

 similar to but smaller than in the species referred to above, which 

 has 20 anterior and 10 posterior teeth in a valve of the same 

 length. Length, 39; height, 22; diameter, 14 nun. (Cat. No. 107688, 

 U.S.N.M.) 



Station 3077, in Clarence Strait, Alaska, in 322 fathoms, mud. 



