56 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



Original diagnosis: Shell compressed, thin, hyaline. Valves nearly 

 equal, with concentric undulations or corrugations, affecting the entire 

 thickness; margins simple; sculpture none, or consisting of fine radial 

 lines on one or both valves, without camptonectes sculpture. Hinge-plate 

 thin and nearly plain ; auricles well developed, unequal ; byssal notch 

 distinct. 



Remarks: The regular concentric undulations of the disks are func- 

 tional and always present, distinguishing Hyalopecten from any other 

 supraspecific unit in the Pectinidae. In Delectopecten obscure and ir- 

 regular concentric folds of the umbonal portion of the disk occur only 

 occasionally, and are not functional. 



The eastern Pacific Pecten neoceanicus Dall differs from Verrill's 

 diagnosis in having the disks reticulately sculptured, an ornamental fea- 

 ture of only specific significance. 



Only four known species are referable to Hyalopecten : the type 

 species (dilectus), biscayensis Locard (Bay of Biscay), neoceanicus Dall, 

 and pudicus E. A. Smith, the type locality of the latter being Marion 

 Island, Prince Edward Islands, South Indian Ocean. 



Cyclopecten (Hyalopecten) neoceanicus (Dall) 1908 



Plate 2, fig. 2 



Pecten (Pseudamusium) neoceanicus Dall, 1908, p. 402, pi. 9, fig. 4. 



Holotype: U.S. National Museum. 



Type locality: Albatross station 4721, SW of Galapagos Islands 

 (8° T S, 104° 10' W), in 2084 fathoms, globigerina ooze. 



Original description : Shell small, thin, brownish white, concentric- 

 ally undulate and with both valves similarly reticulately sculptured, equi- 

 valve, somewhat equilateral; beaks (showing small, polished prodisso- 

 conch) small, pointed, the left one rising a little higher than the other; 

 hinge line straight, ears subequal, sharply pointed, byssal notch and 

 fasciole distinct, sculpture like that of the disk; surface of the valves 

 with about four easy, wide, rounded undulations; surface sculptured 

 with numerous fine, squarish, radial and concentric threads, subequal 

 and subequally distributed, so that the meshes are mostly square, without 

 nodulation at the intersections, and the interspaces are wider than the 

 threads ; disk short, kite-shaped, a little produced in front, suddenly con- 

 stricted at the margin ; interior white, polished, the umbonal cavities 

 extending under the hinge line; the pit and resilium small. Alt. 12.0; 

 lat. of shell, 12.0 ; of hinge line, 7.7 ; diam. 4.0 mm. 



