METAZOA MOLLUSCA. 207 



and the right valve is smaller and flatter than the left. 

 The hinge has one or two transverse or cardinal teeth. 



Malleus when young (No. 399 a) somewhat resembles 

 the adult Avicula. The hinge line is long with the beak 

 at one end. Only one wing is developed. Later growth 

 takes place on the other side of the beak (No. ^gb) and 

 the result is a wing in masquerade. The adult (No. 400) 

 for obvious reasons is familiarly known as the "hammer- 

 oyster." 



One of the ancestral forms of the oyster was Gryphaea 

 arcuata Lam. (No. 401). The large deep lower valve and 

 the small lid-like upper valve, the curved beak and the 

 lines of growth are all well preserved in the fossil. 



The development of the oyster from the embryo to the 

 adult is given in PI. 402, figs. 1-19 ; figs. 1-3, Ostrea 

 edu Us \Jx\n.\ figs. 4-19, our common species, O. virgin- 

 tana Listner (=O. virginica Gmel.). The segmentation 

 and earliest embryonic stages are omitted, although like 

 the later stages, they illustrate accelerated development. 

 In the embryonic stage (figs. 1-3) the shell is symmet 

 rical with a straight hinge line (fig. i) situated on the 

 dorsal side. The valves are equal (fig. 2) with only 

 slightly developed umbos. In this condition the shell 

 bears a striking resemblance to the equivalved bivalves 

 already described. 



At this time both the mouth (fig. 3, m) and anus (fig. 3, 

 a) are situated ventrally, while there is but a single 

 muscle, the anterior adductor (fig. 3, a, a) . A ciliated 

 velum (fig. 3, v) still persists as the little oyster swims 

 about freely in the sea. 



The stages of development between the one repre- 

 sented by figs. 1-3 and that shown by figs. 4, 5, have 

 never been figured or described. The latter (figs. 4, 5) 

 represents our common species, when it has completed the 

 embryonic or prodissoconch stage, and has fastened itself 

 by the edge of the left valve (fig. 5), using the reflected 

 margin of the mantle (fig. 5, m) to accomplish the work. 



