OF CONCHOLOGY. 339 



The A. digitalis, Esch., I regard as an extreme form of the 

 normal Ore^ona type. It differs in being much smaller, with the 

 apex very near the anterior margin, and the color having a large 

 share of light ash outside, and a yellowish cast within. It is 

 found abundantly in the Northern localities. 



The A. radiata {=ancyhis) of Esch. is probably a variety of 

 the Northern digitalis form, But Philippi asserts that the 

 types are Chilian. His authority, however, is not irresistible 

 evidence, as the South American shells of the Oregona form 

 can scarcely be distinguished from their Northern allies: v. 

 infra. 



This species is, by far, the most variable of the limpets on 

 the coast. No one seeing specimens of digitalis and textilis 

 side by side, would believe that they could have a common 

 origin ; perhaps they would not even be regarded as congen- 

 eric. If each form had kept pretty closely to its own type, 

 the intermediate specimens might have been regarded as hy- 

 brids; but the transitions appear too numerous and gradual 

 for that hypothesis. Sea the remarks on Lottia gigantea. 



AcM^A SPECTRUM, (Nutt.) Eeeve. 



=Fig. 456, 4o6a, FJxpl. Exped. Mollusca, quasi Lottia scabra^ 

 Gld.: [non diagnosis, nee, Jig. 4:66b, = A. persona^ var.) 



This shell is, in general, so easy to recognize, that it stands 

 without synonym, with the unfortunate exception of a soli- 

 tary shell having been figured by mistake, under A. scabra, 

 Gld., (non Nutt.); that species being otherwise a variety of A, 

 persona. A. spectrum is a Southern form, ranging from Lower 

 California to Monterey, but not found in the Vancouver dis- 

 trict. Normally it is solid, rather depressed, with from 20-30 

 very strong, rounded ribs, not evanescent anteriorly, the in- 

 terstices being occupied by intercalary riblets. The color is 

 white, with fine lines of brown (not striped, as in pelta and 

 persona,) between the principal ribs, which delicately dot the 

 otherwise uniform white margin. The habit of growth and 

 consequent varieties of sculpture, resemble those of the flat- 

 tened SiphonariEB, Sometimes the principal ribs are rather 

 sharp, palmating the margin. Occasionally they are small 

 and crowded, becoming faint at the margin, when the shell 

 presents the internal aspect of A. mitella; at other times as- 

 suming that of Patella jjediculus. Generally the apex is at 

 the anterior third; rarely at the anterior fourth, with very 

 elongated outline ; but sometimes it is nearly central, with a 

 rounded shell. In this species also there is occasionally found 

 a var. textilis; when the ribs become faint and distant, the 

 color-lines run into network, and the shell is of a thinner tex- 



