148 MAZATLAN BIVALVES 



203. AVICULA STEKNA, GrOuld. 



Proc. Bost. Soc. Nat. Hist. Nov. 1851, vol. iv. p. 93 : Hex. $ 

 Gal. Shells, p. 31, pi. 16, f. 7. C. B. Ad. Pan. Shells, p. 250, 

 no. 392. 



Avicula Atlantica, Menke, Zeit.f. Mai. 1847, p. 187, no. 45 ; 

 non Lam. 



''Has the general appearance of A. semisagitta, Lam. but 

 the wing is less oblique, and curves directly into the cauda 

 without any sinus ; the byssal sinus is deeper and much more 

 acute, making a trapezoidal auricle. It most nearly resembles 

 a Florida species ; but in that the byssal angle is obtuse, the 

 auricle triangular, and the cauda shorter." Gould* The hinge 

 line is generally extremely long, but sometimes so short that 

 the anterior are rather larger than the posterior auricles. Shell 

 very insequivalve. Outer layer of shell dark chocolate, rarely 

 slightly rayed with white. Surface almost smooth, with very 

 fine concentric lines of growth. Epidermis in fine strongly ser- 

 rated, closely folded laminae. Hinge teeth, card. 2-1, lat. 1-1. 

 The pallial line is traceable from the large bilobed posterior 

 adductor to the small anterior, situated just below the cardinal 

 teeth. A peculiar species of Balanus is commonly found on 

 this shell, and on the related W. Indian species. Many speci- 

 mens of Discoporsea intricata, and eggs of G-asteropods were 

 also found attached. The smallest sp. is *05 in length ; the lar- 

 gest 1'24. A normal sp. measures long. '98, lat. 2 '5, alt. '42. 



One with short hinge T23, 1*7, '59. 



Hob. Panama, Col. Jewett, Do. ; very rare, attached to a 

 small species of G-orgonia, at the low water mark of the 

 spring tides ; C. B. Adams. Mazatlan ; not uncommon ; 

 Col. 



Tablet 676 contains a minute valve, *05 long, with the lateral 

 tooth conspicuous. 677, 7 young pairs of different sizes, (one 

 with egg cases) and a valve shewing spotted umbo. 678, 4 sp. 

 adult. 679, 3 do. shewing interior. 680, 4 sp. different ages, 

 with the 'ear,' 'wing' or 'tail' very much shortened; the 

 larger with Discoporsea intricata. 



* Dr. Gould's * wing"/ appears to be the body of the shell ; his 'cauda' what 

 is by others called the wing or the ear. 



