20 Panama Shells. INTRODUCTION. 



neath, accommodate a great diversity of species : in some 

 parts, flats of mud or of sand prevail. 



On the west of the city is a very broad gently sloping 

 beach of fine sand, where Oliva, Tellina, Donax, and Artemis 

 abound. Then proceeding westward we find alternating 

 ledges of trachytic and basaltic rocks, sand beaches, and 

 broad muddy or sandy flats, with groves of trees a little above 

 half tide level. Here during the recess of the tide, the col- 

 lector finds an agreeable shade, and gathers Purpurse and 

 huge Littorinse from the trees, and numerous species of Ve- 

 neridae, of Columbella, the little and elegant Yeritina picta, 

 and sometimes the massive Area grandis, among the sticks 

 and moss-like Algae beneath. Three miles west of Panama 

 a Rio Grande enters the bay, and is bordered by impenetra- 

 ble thickets of mangroves and not impenetrable ooze. 



On the east side of Panama is a steep sand beach, atthe 

 bottom of which the flats consist of an impalpable mud of a 

 creamy consistence. On this fluid, Marginella sapotilla crawls 

 or rather glides rapidly about. Between two and three miles 

 to the East, there are ledges of smooth basaltic rocks, with 

 abundant Littorinae, Fissurellae, and Siphonariae. Here too 

 are marine groves, rising from a stony surface, by the side of 

 which a small rivulet comes in. At high water mark is a man- 

 grove thicket, beneath which in near proximity we find the 

 Potamides, Area?, a large Cyrena, Potamomyse, and the ele- 

 gant Auricula concinna, and over head is Littorina pulchra, 

 but almost as rare as beautiful. A little farther up. where the 

 water is nearly fresh, occurs the Neritina G-uayaquilensis, 

 whose thin depressed elliptical shell attests its fluviatile ha- 

 bits, unlike the solid ellipsoidal shell of the marine N. picta. 



This diversity of station was multiplied by the high tides. 

 In the office of the Pacific Mail Steamship Co., we saw it 

 recorded on a tide table, that the extreme difference of high 

 and low water had amounted to 28 feet. The ordinary tides 



