SEA-FISILERIES LABORATORY, 34,7 
Microscopic Structure. The observations of Jackson 
(3) on the earliest shell of Pecten crradians show that the 
“ prodissoconch ” (the completed first-formed shell) has a 
homogeneous and laminar structure with fine concentric 
lines of growth, no indications of the byssal notch, and is 
dimyarian. 
The byssal notch arises in the next stage, the 
“ dissoconch,’” which is sharply separated off both in 
structure and shape from the early shell, for a thin layer 
of prismatic cellular tissue was observed in the right valve 
extending over the whole shell. There are no ears nor 
pheations of the shell at the early dissoconch stage, though 
they appear very soon after, and this is interesting 
because the Devonian Pterinopecten, an ancestral genus 
transitional between the Aviculidae and the Pectinidae, 
also shows but slight development of ears. 
It is very difficult to cut sections of the adult shell 
owing to its brittle nature; but I have been able to 
examine a transverse section cut along the antero-posterior 
diameter of P. opercularis and a section along one of the 
ribs, that is, along the dorso-ventral diameter of 
P. maximus (right valve). 
The structure of the shell is practically the same in 
both species, but P. maaimus is much coarser than 
P. opercularis. 
The sections differ considerably in appearance from 
those of Anodon, Mytilus and Cardiwm, and one cannot 
trace the three typical layers—periostracum, prismatic 
layer and nacreous layer. The first appears to have been 
worn away in these adult shells, though traces of it may 
be seen in the hollows. The calcareous structures seen 
probably represent both the prismatic and nacreous layers, 
but the crystals are not laid down as prisms, neither can 
two definite layers be made out. The shell is composed 
