26 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



Type locality : 5.5 miles off Long Point, Santa Catalina Island, 

 California, in 400-430 fathoms, gray-green mud; Hancock station 1613- 

 48; October 2, 1948. 



The species also occurred at 16 additional Hancock stations off 

 southern California; of the 17 stations, 13 were in the San Pedro Basin 

 (between the mainland and Santa Catalina Island), 2 just north of the 

 basin and 2 just west of the island. 



Remarks : This is the thinnest and most fragile species of Pectinidae 

 known to the author. The reflexed ventral portion of the right valve is 

 even thinner than the rest of the disk, and it is impossible to separate the 

 valves without that portion breaking off. 



On the left valve of many specimens a deep water serpulid is found : 

 Protis pacifica Moore (1923, p. 253). From one to five specimens may 

 be living on a single valve, "... usually directed so that the oral aper- 

 ture of the annelid is at or near the siphonal end of the mollusk." (Hart- 

 man, 1955a, p. 52) 



Geographically this species seems to be unusually restricted, for the 

 stations at which it was found were all within an area bounded by north 

 latitudes 33° 14' and 35° 54' 09'^ west longitudes 118° W 58'' and 

 119° 10' 15". Apparently it does not range very far beyond the above 

 limits, for it has not occurred in any of the extensive collecting by Han- 

 cock expeditions in adjacent areas. 



Its bathymetric range is also rather restricted, the minimum depth 

 recorded being 400 fathoms, the maximum 620. 



The 13 San Pedro Basin stations were all located in an area of 

 impoverished fauna which was described and discussed by Hartman in 

 1955b. At the stations in that area the species was found at from 400 

 to 495 fathoms in gray-green mud, associated with foraminiferans, Protis 

 pacifica, two or more species of the annelid Phyllochaetopterus and, 

 occasionally, glass sponge. At the other four stations it occurred at from 

 445 to 620 fathoms in graj^-green mud, associated with foraminiferans 

 and various annelids. 



Cyclopecten subhyalinus (E. A. Smith), known only from off south- 

 ern Chile, is similar to this species, but differs in being less compressed, 

 not having a convex fold on the anterior auricle of the right valve, having 

 distinct radial lirae but not concentric ridges on that auricle, radial 

 and concentric ridges over the entire surface of the left valve, and pos- 

 sessing a deeper byssal sinus. 



Hancock Expeditions Collecting Stations: 



