INTRODUCTION. 



Panama Shells. 21 



are about sixteen to twenty feet. The difference be- 

 tween the spring tides and the neap tides amounts to from 

 four to six feet. Hence at the low water mark of two or three 

 tides, once each fortnight, species of shells, as well as of 

 Echinoderms and other animals, may be obtained, which can- 

 not be procured at any other time except by dredging. Other 

 species live where every tide will leave them exposed to the 

 air for a short time. All the way from low water mark, up 

 to the ledges of rocks where some species of Littorina live out 

 of the reach of the highest tides, species are found, most of 

 which are limited to a very narrow vertical zone. The sur- 

 face in front of the city, which is left bare by the recess of 

 the tide, extends out half a mile or more. Thus over several 

 square miles the secrets of the ocean may be explored. 



The bay of Panama contains several beautiful islands, 

 which rise as steep mountains directly from the water's edge. 

 One of these is Taboga,* 12 miles from -Panama, from and to 

 which a small American steamer runs daily. We made two 

 excursions of three days each to this island. The shores are 

 mostly steep, but sand beaches, ledges, and enormous frag- 

 ments of rocks, afford many favorable stations for the Mol- 

 lusks. At high water, a small island, on the northeast side, 

 is cut off from the principal island, but at low water a high 

 ridge of sand unites them. Here the elegant Conus Maho- 

 gani, Strombus Peruvianus, Natica otis, Corbulaovulata, &c., 

 were found. 



These varieties of station enabled us to find a multitude 

 of species in their proper haunts. But in addition to this ad- 

 vantage, the hermit crabs (Paguridae) swarmed here, as 

 elsewhere in tropical regions. Some of the smaller species 

 have the habit of congregating in heaps of many hundreds 



* This name is sometimes -written, erroneously -we believe, Tabago ; and 

 sometimes, in the publications of the London Conchologists, Saboga. 



