COMPLETE GRAU : PECTINIDAE OF THE EASTERN PACIFIC 21 



Additional descriptive notes: This species is very thin and nearly 

 translucent. The fine ridges near the ventral margin of the right valve 

 are not always present, and between the concentric ridges of that valve 

 are very minute, irregular and discontinuous radial striae. The ribs of 

 the left valve are very low and result from faint plications of the disk; 

 the interspaces are not always smooth, some having one to three faint 

 rows of scales near the ventral margin. The posterior auricles are ex- 

 tremely short, being a little less than half as long as the anterior. 



Remarks: Although strongly resembling Propeamussium alaskense 

 (Dall) in some respects, this species differs primarily in having no 

 internal lirae and much smaller auricles, secondarily in having the left 

 valve faintly plicate, the rows of scales less numerous, and the scales 

 slightly larger. 



The absence of internal lirae would seem to disqualify Propeainus- 

 sium davidsoni from inclusion in Parvamussiuni, but its other features, 

 as well as its close relationship to Propeamussium alaskense, justify the 

 inclusion. The lirae are often completely absent in P. hoskynsi Forbes, 

 and in P. alaskense they are often nearly obsolete, particularly in Jap- 

 anese specimens. Among externally-ribbed species of Pectinidae an 

 analogy is seen in Pseudamussium tigerinus (Miiller), (1776, p. 248) ; 

 that species is usually smooth, but frequently specimens are found having 

 strong plicate ribs. 



Geographical range: Reported only from the Bering Sea. 



Geochronological range: Reported from the Recent only. 



Bathyinetric range: Recorded in 274 to 351 fathoms. 



Ecological data: Found in sand or mud. 



Genus CYGLOPECTEN Verrill 1897 



Cyclopecten Verrill, 1897, p. 70. Type species: Pecten pustulosus Ver- 

 rill, 1873, p. 14; type locality: near Georges Bank (off New- 

 foundland), in 150 fathoms. [Massachusetts to Newfoundland; 

 99 to 547 fathoms.] 

 Cyclochlamys Finlay, 1926, p. 452. Type species: Pecten transenna 

 [sic] Suter, 1913, p. 881, pi. 52, fig. 3; type locality: "near the 

 Snares, in 50 fathoms." [South of South Island, New Zealand.] 

 Original diagnosis: Shells thin, rounded, scarcely oblique, with 

 symmetrical auricles and simple margins. The two valves are quite un- 

 like in sculpture. The right valve is a little flattened and upturned at 

 the flexible margin, so as to fit tightly against the upper valve. The thin 



