MARINE MOLLUSKS — MACGINITIE 155 



upturned in Y. scissurata. Even in specimens only 5 to 10 mm. long, 

 these differences are apparent and consistent. 



A specimen 30 mm. long from the Sea Horse Islands has 7 or 8 

 growth cessation lines, one 23 mm. long has 6 or 7 lines; one 30 mm. 

 long from the Shumagins has 6 or 8 lines, and one 25 mm. long has 

 5 or 6 such lines. 



Distribution: Point Barrow to Monterey Bay; and in the Pleis- 

 tocene at San Pedro, Calif. 



Suborder Isodonta 



Family Pectinidae 



Genus Chlamys Roding, 1798 



Chlamys islandica (Miiller, 1776) 



Pecten islandicus Miiller, 1776, p. 248.— G. Sars, 1878, p. 16, pi. 2, fig. 2.— Old- 

 royd, 1924, p. 54, pi. 8, figs. 1, 2.— Morris, 1947, p. 18, pi. 11, fig. 7; 1951, 

 p. 26, pi. 11, fig. 7. 



Chlamys islandica Morris, 1952, p. 17, pi. 4, fig. 4. 



This species was taken only sparingly. Because of marked varia- 

 tions in measurements, the specimens are listed in detail in table 4. 



Other material examined: Several entire shells and about 30 

 valves from localities ranging from Point Barrow through Bering Sea 

 to the Aleutians and the Shumagins; about 15 shells and as many 

 valves from Greenland, Labrador, Newfoundland, Nova Scotia. 

 Maine; 2 from Vads0, Norway, and 1 from Finmark, Norway. 



Discussion: These shells vary from light to heavy, and the sculp- 

 ture varies from closely set, fine, rounded or inverted V-shaped, radi- 

 ating ribs covered with closely set scales to broad, smoother, more 

 widely spaced ribs, usually with a single secondary rib in the wide 

 groove between the primary ribs (pi. 19, fig. 4). The primary ribs 

 of the heavy shells characteristically divide, the resulting two ribs 

 then continuing close together like a double rib. In some of the more 

 highly sculptured specimens with prominent scales, three or more ribs 

 may lie closely together, forming a ridge, with from one to three sec- 

 ondary ribs in the grooves between the ridges. One specimen from 

 Vads0 resembles one from off Cape Cod and one from off Nova Scotia 

 in that the ribs are scaly and tend to group into ridges. Except that 

 the ribs are more closely set and scalier, another specimen from Vads0 

 resembles the Point Barrow specimens in having broad primary ribs 

 that divide and in having a fine secondary rib between the primary 

 ribs. Not all of the ribs in any one shell necessarily divide. A speci- 

 men from Massachusetts Bay resembles the Point Barrow ones more 

 than specimens examined from Iceland. 



473771—59 7 



