EXPLANATION OF PLATES. 



All figures of oysters, except Fig. 1, are of 0. virginiana Lister. 

 All the figures, with the exception of Fig. 1, were drawn directly from na- 

 ture. The figures of microscopic specimens were copied from camera lu- 

 cida drawings. All oysters, excepting Figs. 1, 7 and 8, were collected at 

 Buzzards Bay, Massachusetts. For Fig. 19 I am indebted to the kindness 

 of Mrs. J. W. Elliot. Figs. 2, 5, 6 and 14, were drawn by Mr. F. W. Cobb. 

 All other figures were drawn by the author. 



PLATE IV. 



Fig. I. 1 Young European oyster (0. edulis Linn). Antemonomy- 

 arian stage, viewed from the left side. a. ad, Anterior adductor, which 

 alone exists at this stage ; m, Mouth ; as, (Esophagus ; a, Anus ; i, Intes- 

 tine ; st, Stomach ; 11, Left lobe of the liver ; v, Velum ; rs, ri, Superior 

 and inferior muscles, which retract the velum into the shell, sh. The 

 shell is in its early prodissoconch stage; the hinge, h, is straight. 



Fig. 2. Dimyarian stage, viewed from the right side, as when attached. 

 a. ad, Anterior adductor muscle; p. ad, Posterior adductor; m, Mouth; 

 ce, GEsophagus ; pi, Palps; a, Anus; g, Gills. The shell has completed the 

 prodissoconch stage, and a slight spat growth, s, exists along the lower 

 margins of the valves. The straight hinge line of fig. 1 is lost. The devel- 

 oped umbos point posteriorly. Magnified about 130 times. 



Fig. 3. Anatomy of very young spat, viewed from the right side. p. ad, 

 The single adductor, the morphological equivalent of the posterior ad- 

 ductor of fig. 2; m, Mouth; pi, Palps; a, Anus; g, Gills; mt, Margin of 

 mantle. The lines of growth of the shell are shown in the left valve of 

 the prodissoconch, p, but are omitted in other parts of the shell for the 

 sake of clearness ; p', Right valve of prodissoconch ; s, Right valve of the 

 spat growth. Magnified 120 times. 



Fig. 4. Margin of a living gill from a little older stage of oyster than 

 fig. 3. 



Fig. 5. Fully developed prodissoconch, viewed from the right side, 

 growing on a glass slide. I, Left valve; r, Right valve. Magnified 130 

 times. 



Fig. 6. Prodissoconch, viewed from the anterior end. I, Left valve; 

 r, Right valve ; s, Spat growth, which has begun along the margin of the 

 prodissoconch valves. Magnified 130 times. 



1 After Huxley, reduced one-half from his figure, published in Eng. 111. Mag. See 

 No. 8 and again in Bull. U. S. Fish Com. Rep., No. 23. 



(550) 



