Geological Papers. 177 



from N. truncata Gabb by its size, its larger and more pronounced 

 umbones, by the large concave area posterior to umbones, and by 

 the low ridge on the posterior dorsal front of each valve, back of 

 which extends the posteriorly curved sinus from umbo to ventral 

 margin, and which continues to widen as it approaches the latter. 

 It resembles some, of the Japanese species, both living and Ter- 

 tiary, very much, but has the beaks pitched and curved more 

 backward. 



Only two specimens were obtained from the upper series of the 

 Clallam formation at Gettysburg, and one from the Neah Bay sec- 

 tion of the same formation. 



Genus Yoldia Moller. 



"Shell oblong, slightly attenuated behind, compressed, gaping, 

 smooth or obliquely sculptured, with a dark-olive shining epider- 

 mis; external ligament slight ; cartilage as in Led i; pallial sinus 

 de^p. Yoldia myalis Couth, is a characteristic species." (Arnold.) 



4. Yoldia impressa Conrad. Plate I, fig. 4, 

 Nucula impressa Con., Wilkes's Explo. Exped., vol. X, p. 726, pi. 18, fig. 7. 

 Yoldia impressa Meek, Sm. Check List, Miocene. 

 Yoldia cooperi Gabb fin part), Proc. Cal. Acad. Sci., 1865, p. 189. 

 Yoldia cooperii Cpr., Cooper (in part). Enumerations of West Coast Mo I- 



Conrad's description r^'"^ "Oblong ovate, convex, with regular 

 concentric impressed lines. Anterior extremity rostrate, slightly 

 recurved, extremity truncated; ligament-margin arcuate, slightly de- 

 clining; rounded behind. Beaks submedial. Basil margin arcuate, 

 slightly contracted near the anterior extremity." 



The fine lines of the surface are neat, but closely crowded. The 

 apical angle is about 160 degrees. 



This species is very variable. The length varies from one to two 

 and one-half inches; the breadth of the mature shells is usually 

 about one-half inch. 



The shells of this species are much more gibbous than the shells 

 oi' Yoldia Gonperi, and are also much shorter. 



This species was found in the Gettysburg shales of the Oligocene- 

 Miocene series; also in the lower and middle series of the same 

 formation at East Clallam. The shells in the Gettysburg series 

 were found to be larger than those obtained at East Clallam. 



The shell figured is from the Gettysburg formation. 



Miocene: Astoria, Ore. (Conrad); between Martinez and Monte 

 Diablo, Cal.; Astoria, Ore. (Cooper; Gabb). 



Oligocene-Miocene: Gettysburg, East Clallam, Wash. (Reagan). 



35. Loc. cit. 



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