92 



University of California Publications. 



[Geolog-s 



Gabb gave the following description: "Shell thin, broad; 

 beaks prominent, incurved, approximate, slightly twisted ante- 

 riorly; hinge line short: ends and base pretty regularly rounded, 

 posterior basal portion a little the most prominent ; area very 

 narrow, slightly sunken. Surface marked by about 25 prominent 

 square ribs, with flat, equal interspaces; these ribs are each 

 marked by a more or less distinct median groove, and crossed by 

 pretty strong concentric lines of growth, breaking up the surface 

 into a beautiful beadin»'. Hinge straight, composed of numerous 

 fine teeth, very small and irregular in the middle, longer and 

 slightly oblique toward the ends." 



This shell is ordinarily elliptical to quadrate in form and 

 inequilateral. The ratio of length to height is about lVi to 1. 

 The size of average adults is about 77 x 63 mm. The beaks are 

 prominent, close together, wide, centrally located, and strongly 

 incurved over a ligament area which though wide is not flaring. 

 The elevation of the ligament area and width of the beaks makes 

 the upper margin appear strongly rounded. 



There are 25-27 ribs. These are flattened, considerably wider 

 than the interspaces and marked by a very distinct median 

 groove, to which are added toward the margin in old individuals, 

 two subsiduary grooves, and later in the largest specimens an 

 additional pair. In these latter the ribs toward the margin are 

 often several times as wide as the intervening spaces. Crossing 

 the ribs are numerous closely arranged lines, giving the shell a 

 beautiful beaded effect, especially well shown in younger indi- 

 viduals. 



This species becomes very large, some individuals being 

 observed 114x76 mm. The smaller individuals showing the 

 heavily beaded ribs have generally been considered as Gabb's A. 

 sulcicosta, the larger being referred to Conrad's A. triUneata. 

 A number of fine specimens from Capitola, and also a good suite 

 from Russian River, show the apparent specific differences to be 

 merely those of age. 



Occurrence and Associated Fauna. — Conrad gives Santa Bar- 

 bara as the type locality for A. triUneata, and lists the following- 

 fauna from that place : 



