Dendraster ashleyi, specimen number 4330-77088 from Chula Vista. Dendraster excentricus, from Crown Point in Mission Bay.

Dendraster ashleyi
Dendraster excentricus
Sand dollars
Dendrasteridae

Time
Pliocene and Pleistocene Epoch

In Our Region
Dendraster ashleyi fossils are common in Pliocene-age deposits in Chula Vista. A subspecies, Dendraster ashleyi ynezensis, is common in the strata beneath downtown San Diego and in the region east of the city. This species was first described and named, without description, by Arnold. Dendraster excentricus is common as a fossil in Pleistocene deposits in Mission Bay and resembles modern D. excentricus, although it tends to be larger than the living species. This species resembles Dendraster ashleyi, except that it has the ambulacral radii positioned much closer to the center of the test than is the pattern of the Pliocene species.

Description
The sand dollar, Dendraster excentricus, is a familiar sight on California beaches. The white, circular shell seen by most people is the test, or exoskeleton of the animal. Rarely, a specimen may be found with a brownish or purplish fuzzy covering of dense, tiny spines. These individuals are recently dead or living specimens. If you observe a wave of movement passing over the layer of spines, the specimen is still alive and should be thrown back into the ocean. On the upper, convex surface of the sand dollar test is a flower-like pattern with five "petals" that mark the ambulacral radii, through which the tube feet protrude. Tube feet are not involved in feeding--the mouth is the neat, round hole in the center of the other, flat side of the test. Sand dollars live partly buried in sand at an angle in calm water. In rougher water the animals lie flat, either at the surface of the sand or partly buried. Subtidal populations sometimes exceed 625 animals per square meter of bottom.