PANAMIC-PACIFIC PELECYPODA 29 



on the Mytilidae by Tron Soot-Ryen and the other on the Arcidae by 

 Helen Rest, both papers reporting on collections of the Allan Hancock 

 Pacific Expeditions. Several families of the bivalves, such as the Arcidae 

 (31 species), Mytihdae (25 species), Veneridae (62 species), Tellinidae (54 

 species), and Donacidae (36 species) have more species than they do in the 

 Western Atlantic. 



As this class of the MoUusca will be treated at some length in the 

 systematic section of this work, only a few of the more striking features 

 of Panamic bivalves will be noted here. Many species could be cited as 

 being characteristic of the Panamic-Pacific fauna as known today, but 

 as in the case of the gastropods, the larger number of these forms have 

 identical or closely related species in the Tertiary faunas of the Caribbean 

 and West Indian region. 



The Nuculacea of the Panamic region, as elsewhere, comprise a large 

 number of small forms of no particular interest with the exception of the 

 following: the genus Adrana with its peculiar, flattened, bladelike shell 

 (some of which attain large size) are particularly characteristic of the 

 Panamic fauna, and several species appear in numbers on certain favorable, 

 sandy beaches; Nuculana {Politoleda) polita, notable for its large size and 

 curious surface markings, is always common on the beach at Old Panama. 

 Among the true taxodont moUusks, the Panamic Arcidae deserves notice; its 

 species are numerous and occupy a wide variety of ecological stations. 

 Amongst the true arks. Area pacijica and A. mutabilis, are relatively less 

 common than their twin forms in the Caribbean fauna. The most important 

 of all Panamic bivalves in the food economy of the region are the two 

 Anadaras, A. tuberculosa and A. grandis. Both species are intimately associ- 

 ated with a mangrove mud environment. A. tuberculosa, and its near ally 

 A. similis, live buried in the mud amongst the mangrove roots; A. grandis 

 in the nearby esteros or mud banks just outside. The range of A. grandis 

 and A. tuberculosa is, therefore, coextensive with that of the mangrove 

 itself (Rliizophora mangle). The most interesting of all Panamic arks is 

 Litharca lithodomus, a true rock borer, in which the shell has become 

 greatly lengthened or pholad in shape. The anterior side is much the longer, 

 wedge-shaped at the end while the posterior side towards the open end of 

 the bore is short. This ark drills circular holes in hard layers of sandstone, 

 often six inches or more deep. This curious species is a member of the true 

 Arcinae. Although discovered by Cuming nearly 150 years ago, it long 

 remained one of the rarest and least known of Panamic mollusks. Restricted 

 to the Pacific Coast today, is the unsymmetrical Noetia reversa but with 

 almost identical forms in the Miocene of the Caribbean region. Another 

 Noetia is N. olssoni which is closely allied to A^. centrota of the Caribbean. 

 As an additional illustration of the close alliance of the present-day Pacific 

 molluscan fauna with that of the Miocene Caribbean mention may be 

 made of the rare Sheldonella delgada of which the only other known species 

 is S. maoica, common in the Miocene of Santo Domingo. As in most tropical 

 faunas, the Pectens are comparatively few in number. Of these, the only 



