116 THE NAUTILUS. 



been evolved since the Eocene. All of the larger forms have very 

 numerous marginal tentacles on the mantle, corresponding somewhat 

 to the external sculpture of the shell in position, and there is also a 

 second inner row of "guard tentacles" on a raised inner pallial 

 fold. The marginal tentacles are accompanied by a series of well 

 formed pallial eyes, very lustrous when living, arid having a crys- 

 talline lens. These are also arranged according to the ribs of the 

 shell. In some of the deep sea forms there are but few eyes, and in 

 some cases they are not pigmented. Pectens, as is well known, are 

 good swimmers, although the very inequivalve typical forms are 

 rather sedentary. This probably enables them to escape the attacks 

 of crabs, fishes, etc., as well as boring gastropods, for it is certainly 

 true that bored Pectens are rarely seen. 



The nomenclature is thoroughly sifted, and the genera and sub- 

 genera characterized, 27 being recognized, of which 8 are new. 

 The subdivision seems rather minute to one accustomed to the old 

 order of things, but has doubtless been well considered. Of the 

 genera, PECTEN restricted, scarcely occurs on our coasts. AMU- 

 SIUM is represented by a couple of deep sea form?, but the Oriental 

 species is a well known shell. The subgenus Propeamusium contains 

 small species such as Dall's alaskensis. CHLAMYS, type islandicus, 

 is well represented, C. irradians, disloeatus, etc., belonging here to 

 the subgenus sEquipecten. Lis#opecten is a new subgenus proposed 

 for C. hyalinus Poli, and Leptopecten another for C. monotimeris of 

 California. Placopecten is proposed for P. cl'mtonins Say, also as a 

 subgenus of Chlamys. LYROPECTEN contains L. nodosus, subnodo- 

 sus, etc. PECTINELLA is a new genus for P. sigsbei Dall of the 

 West Indies in deep water. CYCLOPECTEN is a new genus for P. 

 pustulosus Ver. and many other small species, mostly of deep water. 

 Hyalopecten, n. gen., type P. undatus Ver., contains small forms pos- 

 sibly related to the fossil genus Syncyclonema. PARAMUSIUM, type 

 P. Dalli Smith, is another new genus. 



Several new forms are described, with useful notes on others, 

 among which we may mention with approval the rejection of Gme- 

 lin's misleading name magellanicus for the well known New Eng- 

 land species. Verrill calls it Chlamys (Placopecten) clintonius Say. 

 A very useful analytical key to the genera is given, and six well 

 drawn plates illustrate important structural features and unfigured 

 species. 



