1888.] 543 [Jackson. 



rived from the periconch of Scaphopods. 1 It is seen from the study 

 of Ostrea that the true shell, that which characterizes the adult, 

 originates with the introduction of the spat stage. The earlier 

 shell is strikingly different. It is the completed shell of the veliger 

 antemonomyarian and dimyarian stages. It is distinctly the ho- 

 mologue of the protoconch and periconch. In the oyster, how- 

 ever, this shell is not single, but double-valved, and, therefore, 

 deserves a distinct name, as it precedes the dissoconch or true shell. 

 I suggest the name prodissoconch 2 or early double shell. A pro- 

 dissoconch similar to that of Ostrea has been found in many allied 

 genera. 



The spat stage in Ostrea virginica as a whole is divisible into 

 five stages. These five stages of growth are laid down one after 

 another, and are more or less clearly marked in every regular grow- 

 ing, unworn, young specimen, fig. 7, plate v. To study these 

 stages to the best advantage specimens should be sought growing 

 in protected places, as the wear of sand and waves destroys their 

 demarcation very rapidly. Some spat which I found in the inside 

 whorls of dead Busycons, and others which I secured on artificial 

 cultch of glass in drain pipes, and on inverted flower pots, retained 

 the markings clearly. 



The true larval or silphologic stages of Professor Hyatt (13) be- 

 gin, according to the classification of that author, with the forma- 

 tion of what Owen called the apex of the conch, or true shell. It 

 is shown above that the spat marks the beginning of the disso- 

 conch (true shell) in Ostrea ; therefore the stages into which it is 

 divisible are here considered as the silphologic stages. 



First silphologic stage (figs. 10, 11, plate v). — The spat growth 

 begins along the lower margin of the prodissoconch valves figs. 2 

 and 6, pi. iv, and throughout life under normal conditions the most 

 rapid growth is in this direction. The growth rapidly pushes up 

 the sides of the valves, and between the hinge areas of the latter, 

 fig. 8, pi. v, lifting them upwards, and separating them. A little 

 to the right of the umbo of the lower left prodissoconch valve, as 



1 Professor Brooks (2) says that the embryonic shell of Anodonta is at first a 

 cup, covering what is to become the dorsal surface of the embryo and is, therefore, ho- 

 mologous with the embryonic shell of Gasteropods and he compares it closely to the 

 first shell of Dentalium. See (16). 



For the relations of the periconch to the protoconch, and the origin of the term peri- 

 conch. See (13). 



2 np6, before; Aio-o-ds, double; Koyxn, shell. 



