31 



and is covered by a thick chestnut colored epidermis, conspicuously 

 corrugated at every part, in vertical wrinkles posteriorly, but in a 

 somewhat divaricate manner anteriorly. The posterior superior 

 portion gives rise to a byssoid fringe. 



Length three inches ; height and breadth half an inch. 



Found in indurated marly clay. Monterey. Maj. Rich. 



This species is so peculiar, and so entirely different from any 

 other of the genus, as to require no comparison. 



AVICULA STERNA (PL XVI. Fig. 7.) 

 Avicula sterna, Gould; Proc. B. S. N. IL, Xov. 1851. IV. 93. 



T. solidula, badia radiis pallidis ornata ; epidermide cinerea serratim fimbriata 

 induta ; apicibus ad quadrantem anteriorem positis; ala semicircular!, mod ice 

 obliqua; dcclivitate umbonali tumida, abrupta ; cauda longissima, gracili; auri- 

 culis magnis, radiatim sti'iatis ; fissura byssali profundissima, acutangulari : 

 margarita argentata, valde iridescens ; limbo fusco-marmorato. 



Shell rather solid, obliquely ovate, triangular, with a very long, 

 slender, acuminate cauda, and well marked auricles : color dark 

 mahogany brown with a few paler radiations; surface compactly 

 striate, covered with a thin, dirty, cinereous epidermis, very deli- 

 cately fringed with radiating series of minute processes, especially 

 in the vicinity of the byssal notch and at margin. Beaks near the 

 anterior fourth, prominent, oblique, the posterior umbonal slope 

 tumid, and in the left valve limited by an obtuse groove, a sub- 

 marginal angle along the cauda ; auricles radiately striate. Auricle 

 of right valve trapezoidal, widening towards tip and obliquely trun- 

 cate ; byssal notch deep and acutangular; auricle of left valve sub- 

 triangular, as broad as long, the byssal fold prominent. Hinge with 

 a well marked cardinal and lateral tooth in left valve; the cardinal 

 tooth in right valve double. Cavity of beaks very decided. Nacre 

 silvery and highly iridescent; cretaceous margin like tortoise shell. 



Length of hinge margin two and a half inches; diameter of wing 

 one and one fourth inches ; breadth five eighths of an inch. 



Inhabits Mazatlan ? Lieut. Green. Panama. Prof. Adams, Col. 

 Jewett. 



Has the general appearance of A. semi-sagitla, but the wing is 

 less oblique and curves directly into the cauda without any sinus ; 



