108 PATELLA-HELCION. 



arrangement), some of which also possess a rhachidian tooth, 

 as Hogg and others have demonstrated. 



P. MEXICANA Broderip & Sowerby. PL 31, figs. 59-62. 



The shell is very large, thick and heavy, oval, conical, with 

 central summit. The dull, soiled white, eroded surface shows]about 

 10 low angles or obsolete ridges, and young shells are rather finely 

 striated (fig. 61). 



The interior is pure white or tinged with flesh-color, having also, 

 usually, some brown or purplish stains. The muscle-scar Js ^con- 

 spicuous, roughened. 



Length 200, breadth 150, alt. 80 mill. 



Mazatlan ! San Bias I Acapulco ! W. Mexico ; also Central 

 America. 



P. mexieana B. & S., Zool. Journ. iv, p. 369. MKE., Zeitschr. f. 

 Mai. 1851, p. 37. REEVE, Conch. Icon. f. 1. CARPENTER, 'Mazat. 

 Cat. p. 199.' Ancistromesus mexicanus DALL, Amer. Journ. [ Conch, 

 vi, p. 266, t. 15, f. 21 (dentition). Lottia gigantea OLD., ms. in 

 B. M. Patella maxima ORB., Voy. Amer. Merid. p. 482, and in 

 B. M. Catal. d'Orb. Moll., p. 53. 



This is the largest living species of limpet, frequently attaining a 

 length of 8 to 14 inches. The animal is black, more or less mar- 

 bled and streaked with white. The shell is often used as a wash- 

 basin in Central America. (See Dall, I. c.) 



D'Orbigny described this species as P. maxima, giving the 

 locality Payta, Peru. 



Subgenus HELCION Montfort, 1810. 



Helcion MONTF., Conch. System, ii, p. 62. GRAY, Guide, p. 176. 

 DALL, Amer. Journ. Conch, vi, p. 276. 



Helcion is composed of limpets differing from Patella s. s. in hav- 

 ing the gill-cordon interrupted in front, the shell cap-shaped, apex 

 curving forward. Two sections are distinguishable : HELCION 

 (restricted), having a strongly convex solid shell with scaly radiat- 

 ing ribs, and PATINA, in which the shell is nearly smooth and 

 thinner. 



Section HELCION s. s. 



The gill-cordon is interrupted over the head, composed of small 

 and filiform strands. 



