124 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 23 



Remarks: One of the major criteria Iredale gave for Juxtamusium 

 was the presence of internal ribs. However, a paratype of /. oblectatum 

 in the present author's collection has none, nor are any present on a 

 specimen of Pectcn rnaldivensis recently examined. The only possible 

 explanation seems to be that under certain lighting conditions and at 

 a certain distance from the shell the reverse surfaces of the external 

 interspaces resemble ribs or lirae of the Amusium type; closer inspection 

 reveals that appearance to be an illusion. 



The most remarkable feature of the type species, a feature not found 

 in any other known species of Pectinidae, is the nearly obsolete ctenolium 

 below the anterior auricle of the left valve; while the right valve has 

 a ctenolium of five or six normally prominent teeth, that of the left valve 

 comprises three or four which are so weak as to be easily unnoticed un- 

 less the shell is carefully examined. 



The other distinguishing criteria for this subgenus are the very thin 

 shell, the large auricles, the very weak plicate ribbing and the simple 

 hinge structure. On phylogenetic grounds these features seem to indicate 

 SeinipaUiuin as the ancestral group. Pccten natans Philippi, although 

 a good deal more inflated and with slightly stronger ribs of varying 

 width, appears to be referable to Juxtamusium. 



Iredale commented, "Smith . . . described a Pecten rnaldivensis 

 which Hedley added to the Queensland list. Hedley's species is here 

 described as new, and the characters are much nearer Amusium than of 

 Pccteriy and therefore a new generic name is proposed." Giving the 

 Australian representative of Smith's species a new name was in accord- 

 ance with Iredale's often-expressed contention that Australian mollusks 

 cannot be conspecific with those living elsewhere, although he did, with- 

 out explanation, violate that self-imposed rule, exceptions among the 

 Pectinidae being the retention (under new genera) of Ostrea pallium 

 Linne and Pccten spcctahilis Reeve. 



Hedley (1909, p. 423) was correct in referring the Queensland shell 

 to Pccten ?naldivcnsis, and qualified his action by acknowledging, "I am 

 indebted to IVIr. C. J. Gabriel for identifying Hope Island specimens 

 with the type in the British Museum." Incidentally, Australian speci- 

 mens of P. rnaldivensis are about one-third larger than those reported 

 from the Maldive Islands, the Gulf of Oman and Mauritius. 



The only known living representatives of this distinctive subgenus 

 are Pecten rnaldivensis and P. natans. 



