15 



Microscopic Structure. Tlie observations of Jackson 

 (3) on the earliest shell of Pecten irradians show that the 

 " prodissoconcli '' (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 

 plications of the shell at the early dissoconch stage, though 

 they appear very soon after, and this is interesting 

 because the Devonian Ptermojjecfen, 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 antern-posterior 

 diameter of P. opercularis and a section along one of the 

 ribs, that is, along the dorso-ventral diameter of 

 P. maximvs (right valve). 



The structure of the shell is practically the same in 

 both species, but P. maxiwus is much coarser than 

 P. opercvlaris. 



The sections differ considerably in appearance from 

 those of Anodon, Mytilus and Cardhim, and one cannot 

 trace the three typical layers — periostraeum. 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 



