8 AXEL A. OLSSON 



the Panamic-Pacific molluscan fauna has remained fundamentally unchanged. 

 The living Panamic-Pacific mollusks are, therefore, a part of a larger 

 relict fauna, once more widely distributed, left isolated in the Pacific zone 

 by the isthmian land uplift of Pleistocene times. This aspect of the origin 

 and affinities of our Panamic mollusks was forecast by early paleontological 

 studies but its full significance and bearing on the Recent faunas were not 

 until now so fully understood. 



In this work the main attention will be focused on the neritic species 

 of the Pelecypoda or those which live along a bottom environment of 100 

 meters (300 feet) or less. This belt of shallow water has sometimes been 

 called the "littoral zone". This is the zone of strong daytime illumination 

 or of light penetration (the Photo zone) suitable for the growth of green 

 plants on which in the ultimate analyses all life in the sea depends. Today 

 the term "littoral" is generally used in a more restricted sense, or for the 

 small area laid bare by tidal changes (intertidal zone) and to the immediate 

 strand line along which a limited number of marine species live. To the 

 larger region of shallow waters extending from extreme low tide to the edge 

 of the continental shelf, the term "neritic" is employed, a name proposed by 

 Haeckel to complement the expression "oceanic" or the environment of 

 blue waters of the open sea. For the same region of shallow water, Ekman 

 proposed the term "shelf zone". 



Modern methods of classification and the naming of animal and plant 

 species (taxonomy) commenced with the adoption of the binomial system 

 of nomenclature introduced by Linnaeus, that of the animal kingdom dating 

 from the publication of the 10th edition of Systema Naturae in 1758. Many 

 excellent works on mollusks had appeared before the Systema but lacking 

 in a uniform method of species naming, their interest to the average 

 student is largely historical. After Linnaeus, the second half of the 18th 

 and the first half of the 19th century, witnessed a feverish activity in 

 exploration and in the naming of new species of mollusks from all parts of 

 the world. In this haste of species making, many new names were proposed 

 without adequate description or illustration so that their subsequent identi- 

 fication was both difficult and uncertain. It was fortunate for the science 

 of malacology that interest in shells was sufficiently broad that the prepa- 

 ration of a fine series of well-illustrated monographs or Iconographs was 

 soon begun. Some of these works on conchology, such as that of Chenu's 

 magnificent Conchological Illustrations in four large folio volumes, are the 

 finest colored illustrations of shells ever produced. At the same time, studies 

 on the soft parts of the animal were started, and the importance of the 

 radula in classification discovered. 



A careful survey of any large family of mollusks will generally show 

 the need for closer generic differentiation and for a revision in classification. 

 Many families of mollusks, as defined at present, are of composite or 

 polyphyletic origin, the genera placed together for convenience or in sharing 

 a few common characters. Such families on close study will be broken apart 

 or divided into smaller categories such as subfamilies which generally means 



