314 



ROBERT TRACY JACKSON ON THE 



fore in favor of the assumption tliat the cement bj whicli the oyster is attached is wliollj 



"With the introduction of the spat stage, a fundamental histological difference arises 

 abruptly in the structure of the shell of Ostrea virgmiana as in O. eduUs, fig. 27. As 

 stated above, the prodissoconch consists of lime infiltrating an amorphous matrix of con- 

 chy olin, but in the initial spat stage the first layer of shell of the right valve is deposited in a 

 "tessellated or prismatic" manner in a horny matrix, as described and figured by Professor 

 Ryder (62, 64). Soon after the formation of the spat stage, the subnacreous, white por- 

 cellanous layer begins to be deposited on the yellowish-brown, prismatic layer, first mak- 

 ing its appearance as irregular blotches in the centre of the valves. These two layers con- 

 tinue throughout the rest of the life of the oyster, and together, though in disproportionate 

 degree, build up the massive adult shell. The prismatic layer always remains thin, as may 

 be readily seen by sections, or by treating an adult with acid, when the remaining conchy- 

 olin of the prismatic layer will separate from the overlying subnacreous layer like a dis- 

 sected sldn. 



It is a remarkable fact that in 

 young Ostrea virginiana ^the 

 prismatic sti'ucture is typical of 

 the upper right valve alone; the 

 lower valve presenting a fine 

 granular structure, as in 0. eda- 

 lis, fig. 29. In later life in Ostrea 

 vtrgiuiana and some other spe- 

 cies, a very thin layer of pris- 

 matic tissue maybe found in the 

 left valve though it is for the 

 most part eroded away, but the 

 same layer is thicker and there- 

 fore better preserved in the right 

 valve. The prisms of the right 

 valve of the adult oyster are not 

 as perpendicular to the surfjice 

 as in the young, fig. 27. There- 

 fore in horizontal section the outline of the prisms presents oblique rather than hexag- 

 onal faces. A similar obliquity of the prisms may be seen in Solemya mediterranea. 



The nepionic or infant stages of molluscs begin according to the classification of 

 stages adopted with the formation of what Professor Owen called the apex of the conch, 

 or true shell. It is shown that the spat marks the beginning of the dissoconch (true 

 shell) in Ostrea; therefoi-e the spat is considered as the nepionic period.^ In my pre- 

 liminary ])aper I said the spat of Ostrea virgmiana is divisible into five stages which were 

 described at some length. I also said that they are more or less clearly marked in every 



Fig. 28. 





i'lii. 



Fig. 29. 



Fig. 27. Young Ostrea edulis. The prodissoconcli is composed of ho- 

 mogeneous and the early dissoconch of prismatic cellular tissue.' The 

 gills are fllanientous. 



Fig. 28. Fragment of border of upper valve, enlarged to show the 

 prismatic structure. 



Fig. 29. Fragment of lower valv-e (after Horst). 



'The relations of structure of tlie prodissoconch and dis- 

 soconch are the same in Ostrea viryiiiiana, also in Perna, 

 recten and Anemia, as described in the text. 



'In my preliminary paper this same period was consid- 



ered as the "silphologic" period. In a footnote to tliis 

 paper Professor Hyatt has given up the use of the term 

 silphologic, and has supplied in place of it the term nepi- 

 onic {vide p. 290). 



