PART I. INTRODUCTION 



1. GENERAL STATEMENT 



This work on Panamic mollusks deals with the genera and species of 

 the Pelecypoda known principally from the southern half of the great 

 Panamic-Pacific zoogeographic province or more precisely from the region ex- 

 tending from Costa Rica southward to northern Peru. This large faunal 

 region in its fullest development begins in the Gulf of California in north- 

 western Mexico and extends southward to northern Peru or along a coast- 

 line of more than 4500 nautical miles. In spite of its great length, its 

 molluscan fauna is remarkably uniform throughout and efforts to divide 

 the province into smaller subregions, not strictly ecological in character, 

 have so far failed. It is in the southern half of the province (which includes 

 Panama) where the Panamic fauna attains its most typical development, 

 and where the majority of its species were first discovered and afterwards 

 named. It is also the region so fully explored by Hugh Cuming at the 

 beginning of the 19th century, the great era of oceanic exploration and 

 conchological description. Shell collecting in Panama and Ecuador may be 

 at times rewarding in the large number of species obtainable at a few 

 special localities where optimum conditions of environment exist, and where 

 large tracts of sea bottom become uncovered during periods of abnormally 

 low or minus tides. 



To the geologist and especially to the invertebrate paleontologist 

 familiar with West Indian, Caribbean, and South American Tertiary fossils, 

 the living mollusks of the Panamic-Pacific region have a special meaning. 

 During the greater part of Tertiary time and more especially in the Miocene 

 and early Pliocene, the tropical West Atlantic and the tropical East Pacific 

 constituted a single zoogeographical province or faunal region (West 

 Tethyan), its species for the most part of Atlantic origin. Channels or sea- 

 ways across parts of northwestern Colombia, Panama, and Costa Rica were 

 then open affording free communication between the Pacific on the one 

 hand and the Caribbean or West Atlantic on the other; hence the Miocene 

 mollusks of the West Indian-Caribbean region are related and in many 

 cases identical with those from rocks of the same age in Ecuador and north- 

 ern Peru. Towards the end of the Pliocene and more fully in the Pleistocene, 

 the isthmian straits were closed by an uplift forming a final land 

 connection between North America and South America. This period of 

 earth disturbance, world-wide in scope, appears to have culminated in the 

 mid-Pleistocene, the time of maximum glaciation in the Northern Hemi- 

 sphere and of a general lowered sea-level elsewhere (see K. K. Landes, 

 1952). It seems likely that large tracts of the shallower parts of the Carib- 

 bean Sea were then drained, resulting in extinction of many species and 

 forcing others to retreat into the deeper portion of the basin. As compared 

 to its richness in the Miocene, the present-day Caribbean mollusks 

 appear strangely modified and greatly impoverished; on the other hand. 



