92 THE NAUTILUS. 



ture of the superficial layer of the shell (often denuded in otherwise 

 perfect fossils), and the sculpture of the valve-margins near the hinge 

 and on each side of it. While not invariable in all specimens, these 

 characters, taken together, will usually enable one to refer the indi- 

 vidual to its proper place. 



The characteristics due to situs may be partially summarized as 

 follows: When a specimen grows in still water, it tends to assume a 

 more rounded or broader form, like a solitary tree compared with its 

 relatives in a crowded grove. When it grows in a tideway or strong 

 current, the valves become narrow and elongated, usually also quite 

 straight. Specimens which have been removed from one situs to the 

 other will immediately alter their mode of growth, so that these facts 

 may be taken as established. When specimens are crowded together 

 on a reef, the elongated form is necessitated by the struggle for ex- 

 istence, but, instead of the shells being straight, they will be irreg- 

 ular, and more or less compressed laterally. When the reef is dry 

 at low stages of the tide, the lower shell tends to become deeper, 

 probably from the need of retaining more water during the dry 

 period. Such oysters are the so-called " raccoon oysters," a name 

 which they get from the visits of that animal at low water to feed 

 upon them. The so-called "raccoon oysters" figured in Dr. C. A. 

 White's Review of the Ostreidte (Ann. Rep. U. S. Geol. Survey, 

 1883, pi. 81-2) are not the reef oysters which first acquired this 

 name, but deep-water specimens which had grown in a place where 

 they were subjected to current action. When an oyster grows in 

 clean water on a pebble or shell, which raises it slightly above the 

 bottom level, the lower valve is usually deep and more or less sharply 

 radially ribbed, acquiring thus a strength which is not needed when 

 the attachment is to a perfectly flat surface which acts as a shield on 

 that side of the shell. Perhaps for the same reason oysters which lie 

 on a muddy bottom with only part of the valves above the surface of 

 the ooze are less commonly ribbed. AVhen the oyster grows to a 

 twig, vertical mangrove root or stem of a gorgonian, it manifests a 

 tendency to spread laterally near the hinge, to turn in such a way as 

 to bring the distal margins of the valves uppermost, and the attached 

 valve is usually rather deep, the cavity often extending under and 

 beyond the hinge margin ; while the same species on a flattish sur- 

 face will spread out in oval form with little depth and no cavity un- 

 der the hinge. 



