This shot shows a huge aggregation of Fingered Limpets (Lottia digitalis) and Southern Fingered Limpets (Lottia austrodigitalis) that covered the surface of a huge rock at Bean Hollow State Beach (coastal San Mateo County, CA). Both species are very similar, the primary difference between them being the pronouncedness of the ribbing on the posterior end of each animal's shell. This would be the end that is furthest away from the shell's apex.

According to most of the authorities on the matter that I've read, L. austrodigitalis has weaker ribbing on the posterior end of its shell than L. digitalis. This gives L. austrodigitalis a smoother appearance than L. digitalis. Refer to the marked-up version of this shot for some useful comparisons.

The range of the two species overlaps in central California; however, the range of L. austrodigitalis has been expanding northwards. In this shot, it actually seems to outnumber L. digitalis somewhat. I have found it fairly regularly on the San Mateo County coast, but do not know if it has gotten north of the Golden Gate yet.

Photographed at Bean Hollow State Beach in coastal San Mateo County, CA, using a basic phone camera.