OYSTERS AND METHODS OF OYSTER-CULTURE. 267 



margin than to the hinge; it is a well-defined scar, kidney- shaped in 

 specimens of ordinary size, but becoming more elongate in very large 

 individuals; in young specimens it is pale, but it afterwards becomes 

 purple or almost black. The left or lower valve is deeply concave 

 Avithin, the upper valve being flat or, usually, slightly concave. The 

 animal portions are large, nearly filling the shell, and the mantle border 

 is comparatively narrow. (Plate v.) 



PACIFIC COAST "NATIVE," OSTREA LURID A. 



The shell of this species is thin and irregular, varying in shape from 

 almost round to elongate elliptical; the surface is sometimes laminated, 

 but is never ribbed; the color is variable, being sometimes purple, 

 sometimes dirty green or gray; the inside of the shell is greenish, 

 sometimes tinged with purple. The muscular impression or scar is 

 purple, but paler than in the eastern oyster, and its greatest length is 

 usually longitudinal rather than transverse; it is situated about mid- 

 way between the hinge and the lips or nibs of the shell, and its ventral 

 margin is usually prolonged toward the hinge. There is rarely a well- 

 defined pit or excavation beneath the hinge, the inner face of the shell 

 sloping oif gently from the ligament. The lower valve is deeper than 

 the upper one, but is rarely so strongly concave as in the eastern 

 species. (Plate vi.) 



REPRODUCTION AND DEVELOPMENT. 

 SEXUAL CHARACTERISTICS. 



In the European oyster the individuals are hermaphrodites — that is, 

 each is both m:ile and female; in the common eastern oyster the sexes 

 are separate, each individual being either male or female, but not both. 



Although the sexes difler remarkably in physiology and minute 

 anatomy, it is not ijossible to distinguish male from female by any 

 known external characters. It is only by an examination of the genital 

 glands, which in the male produce the spermatozoa or milt and in the 

 female the ova, eggs, or si)awn, or by examining the genital j)roducts 

 tliemselves, that the one sex may be distinguished from the other. 



The differences between the ovaries of the female and the testes of 

 the male are explained in the section treating of the anatomy. When 

 the animals are ripe, the distinction of the sexes is most conveniently 

 made by an examination of the genital products. A drop of genital 

 fiuid is extracted from the oyster in the manner described under the 

 head of artificial fertilization (p. 332) and let fall into a glass of clear 

 sea water. If the individual be a ripe female, the drop will break up 

 into a uniformly distributed cloud, which, if examined against a black 

 background, will be seen to consist of separate minute white granules 

 or eggs. If the eggs be unripe, they will remain aggregated in little 

 compound masses. If the specimen examined be a male, the drop of 

 milt will form an irregular, stringy cloud, showing a tendency to drift in 



