BULLETIN 



OF THE 



SoutHern Galllornla flcaflemii o! Sciences 



VOL I LOS ANGELES, CAL, HAY I, 1902. NO. 5 



BO" 

 A Monograph on Pecten Aequisulca-tus, Cpr. 

 CIslSs Pelecypoda.. 



BY MRS. M. BURTON WILWAMSON 



Order Prionodermacea. Suborder Isodonta. Family 



Pectenid^. 

 Pecfen {Plagiocteniuvt) aqeitisulcatus Cpr. Monterey, California, 

 to Todos Santos Bay, Lower California. 

 Shell circular in shape excepting at the hinge margin which is 

 straight, occasionally the posterior part of the shell is somewhat 

 oblique. The ears or auricles of the shell are situated at either 

 end of the hinge margin. The hinge line is narrow and there is 

 an internal cartilage pit which is not broad but rather deep. 

 The beaks are prominent and are close together. There are 

 usually twenty strong ribs on each valve, although occasionally 

 nineteen, and, less often, twenty-two are found*. There are 

 usually the same number of ribs on each valve, but sometimes 

 there is one more on one of the valves. The lines of growth be. 

 tween the ribs are very noticable. The anterior, right auricle is 

 narrower than the ear on the left valve as the byssal opening or 

 notch is found at this point. Directly under this notch in the 

 shell are 4 or 5 points or teeth. Occasionally less and sometimes 

 more than this number are plainly discernable under a magnify- 

 ing glass. The auricles on the posterior side of both right and 

 left valves are equal. The valves are rather convex in shape, 

 the lower, or right one being more ventricose especially near the 

 umbones or beaks. The valves are pointed on the ventral mar- 

 gin and the shell closes excepting at the byssal notch and the 

 ends of the two auricles which are always widely apart, compar- 

 atively. The color of the shell is white or a yellowish tint, al- 



*The size of the specimen does not regulate the number of ribs . I once found 23 ribs on 

 one valve and 22 on the other of a shell less than 2^ inches across, while specimens as 

 large as sJ^ inches averaged 20 or 21 ribs and no valve had over 23 ribs. 



