THK NAUTILUS. 93 



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 undisturbed, the favored individual might attain a much greater 

 age; in which case the lower valve especially took on excessive 

 thickness, and the ca-vity of the shell often became considerably 

 elongated and somewhat hour-glass-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 resiliurn 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 occa- 

 sional 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 case that this habit (as in 0. percrassa Conr.) has 

 attained a constancy entitling it to systematic significance. 



AN ATTEMPT TO DEFINE THE NATURAL GROUPS OF STROMBUS. 



BY GEO. HALCOTT CHADWICK. 



(Plerocera continued.^) 



1. Lip armed with closed spines, 



a. Within smooth, orange, 



(Hepta dactyl as. ) 



Pt. aurantia, lambis, bryonia. 



Distr. : Red Sea and Mauritius to Japan, Australia and Polynesia. 



b. Within wrinkled, violet, 



(MiUipes.) 



Pt. Scorpio, pseudoscorpio, miUipeda, elonyata. 



Distr.: Zanzibar and Mauritius to Japan and New Guinea. 



2. Lip deeply cut into nufinerous open lobes, within finely, deeply, 

 regularly grooved. 



