218 WEST COAST SHELLS 



white, though there is often a dark thread around the 

 edge, and a brown spot in the center. A strange form 

 is sometimes found in which the early growth of the 

 shell seems to have been based on a different plan 

 from that of the ordinary specimen, for it is smooth, 

 brown, and has almost perpendicular sides like the 

 limpets that grow on seaweeds; but after that it sud- 

 denly changes to the ordinary form. It is probable 

 that this was caused by a decided change in the abode 

 of the limpet, perhaps from seaweed to the rock. 



A small, black, conical shell, supposed by Carpen- 

 ter to be an abnormal growth of the young of the last 

 species, is now known as Ac7n6:a asmi^ Midci., the 

 Black Limpet. It is usually found living on the 

 shells of the Black Turban, where its dark color keeps 

 it from being conspicuous. It seems to be quite dis- 

 tinct from the last species, being only a quarter of an 

 inch in length, while the ordinary shells of that spe- 

 cies are fully an inch long, and sometimes more. 



Figure 213 shows the shell of a 

 large Aciricza persona^ Esch., the 

 Mask Limpet. This shell may be 

 pjg 213 distinguished by the position of 



the apex, which is situated very 

 near one end, making nearly all the slope come upon 

 one side, like the roof of an old-fashioned farm- 

 house. The ribs on the slope of the shell are promi- 

 nent but irregular. The outside is gray or mottled, 

 and the inside has varying amounts of brown and 

 white. The shell is high arched, but it is always 

 small and seldom grows to the length of an inch. 



