TRANSACTIONS OF WAGNER 

 676 



TERTIARY FAUNA OF FLORIDA 



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 Ostrcida (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. When 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 margin 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 surface will spread out in oval 

 form with little depth and no cavity under the hinge. 



The average life of the ordinary 0. virginica when " planted" for sale 

 is about four or five years. In prehistoric times when the reefs were un- 

 disturbed the favored individual might attain a much greater age; in which 

 case the lower valve especially took on excessive thickness, and the cavity 

 of the shell often became considerably elongated and somewhat hourglass- 

 shaped, as in 0. contracta Conr., whose characters in typical specimens are 

 distinctly senile, while younger specimens of the same species have the normal 

 form. 



In the hinge of the oyster the resilium occupies the central ridge, while 

 the ligament covers the edge of the depressions on each side of that ridge. 

 The form and relative position of the muscular scar of the adductor is within 

 certain limits a useful character, but its depression below the general interior 

 surface of the valve or its occasional elevation above it, as in Plicatula, is of 

 no systematic value, being merely a corollary of the rate of growth from 

 the various secreting surfaces. The habit of rapid growth, causing a vesicular 

 character of the shell substance, is more pronounced in some species than in 

 others, and in some specimens of a species than in others ; it is rarely the 



