ACM^A. 29 



A. MITELLA Menke. PL 6, figs. 46, 47, 48, 49. 



Shell small, conical, oval, apex in front of the middle; slopes 

 straight or nearly so, surface radiately sculptured with larger and 

 smaller riblets ; color greenish or gray, often encrusted with a white 

 coralline. Edge scarcely crenulated. 



Inside white, with margin regularly spotted with black ; central 

 tract marbled with brown, generally with a bluish-black spot near 

 the center. Length 11, breadth 8, alt. 5 to 6 mill. 



Mazatlan to the Gulf of California. 



A. mitella MKE. Zeitschr. f. Mai. 1847, p. 187. CPR. Mazat. 

 Cat. p. 210 BALL, Am. Journ. Conch, vi. p. 253, t. 14, f. 9 (denti- 

 tion). P. navicula RVE. Conch. Icon. f. 130. 



Small, conical, rather finely radiately ribbed, the inside border 

 regularly spotted with black. 



A. DISCORS Philippi. PL 9, fig. 3, 4, 5. 



Shell oval, solid, depressed, apex near the anterior third ; slopes 

 gently convex. Surface generally encrusted ; having about 10-15 

 strong radiating folds around the apex, usually lost by erosion in 

 adult shells and wholly absent on the later growth of the shell, which 

 is closely, finely radiately striated. Margin very finely crenulated, 

 often dotted with black. Color greenish or grayish- white, with more 

 or less distinct radiating black lines. 



Inside white, often mottled with purple or brown. Muscle-scar 

 distinctly impressed. Length 48, breadth 37, alt. 14 mill. 



Cape St. Lucas and Mazatlan to Panama. 



P. discors PHIL. Abbild., t, 2, f. 6. RVE, Conch. Icon., f. 78. 

 CPR. Mazat. Cat., p. 201. 



This species is peculiar in being puckered around the apex, when 

 not eroded. It is usually encrusted with coralline, algse, etc. 



A. ATRATA Carpenter. PL 7, figs. 61, 62, 63, 64, 65. 



Shell solid, oval, apex at the anterior third ; sculptured with un- 

 equal, irregular ribs, the interstices narrow. On a whitish ground it 

 has black stripes and lines, and the eroded apex is black. 



Inside white ; the border yellowish-gray, tessellated with square 

 black spots, usually bifid ; the central area callous, white, clouded 

 with brown and faint livid-purplish tints. It is excavated anteriorly 

 at the cavity of the apex. Muscle-scar distinct, rugose. 



Length 31, breadth 24, alt. 14 mill. 



