30 ACM^A. 



The young (figs. 64, 65) are flatter, and have very close, acute, 

 unequal ribs, sharply crenulating the margin. 



Cape St. Lucas, L. California. 



A. (f oar.') atrata CPR. Ann. Mag. N. H. 3d Ser., xiii, p. 474. 

 Collisella atrata DALL, Amer. Jour. Conch, vi, p. 225, t. 14, f. 15, 

 15a, (dentition.) 



A perfectly distinct species, allied to A. pediculus, but more 

 elevated, with differently patterned interior. My description and 

 figures are drawn from the types in the Smithsonian Institution 

 collection. 



A. PEDICULUS Philippi. PI. 6, figs. 34, 35 ; PI. 7, figs. 68, 69, 70. 



Shell normally flat, oblong, solid, with ten stout rounded ribs 

 projecting at the margins, of which two are in the axis of length 

 with four on each side ; ribs and interstices radiately striated ; yel- 

 lowish-white generally with more or less of black or brown tortoise- 

 shell markings within, sometimes with black between the ribs. 

 Sometimes the shell is more rounded and the ribs more angular, in 

 which state it might be taken for the young of P. mexicana. 

 Occasionally a few other intercalary ribs appear. In a very few un- 

 usually large specimens, the ribs are nearly obsolete at the margin 

 and the shell is much lengthened. The body mark varies as usual; 

 when plain it is gathered into points as in P. discors. The very 

 young shells appear not to develop the ribs marginally, in which 

 state they might be taken for the young of P. discors. The stout ribs 

 of the adult shell however bear no analogy with the very finely 

 marked surface of the latter with its curiously puckered circum-un- 

 bonal portion. With the young of P. mexicana it has much more 

 close analogies. The largest specimens of P. pediculus however do 

 not at all run into the smallest of P. mexicana. They have all the 

 appearance of being old shells, with the margin narrow and the 

 shape long and irregular, while P. mexicana, as it is traced upwards, 

 displays a very wide semitranslucent margin, and a broad regular 

 shape, with the ribs not rounded and prominent but simply giving 

 an angular form to the shell. Even when very young, they are 

 almost always incrusted with corallinous matter. ( Cpr.) 



West Mexico, Mazatlan to Acapulco. 



P. pediculus PHIL. Zeitschr. f. Mai. 1846, p 21, No. 8. CARPEN- 

 TER, Mazat. Catal. p. 200. P. corrugata RVE., Conch. Icon. f. 132, 



