NATURAL HISTORY SOCIETY OF GLASGOW. 77 



laid aside, as found on Crinoids, decreased — the basal portions of 

 the Crinoid, when recognized in its hydra aspects, having alone 

 obscured several genera of imagined structures. 



II. — On Fresh and Brackish-water Ostracoda, chiefly from the 

 West of Scotland By Mr. David Robertson, F.L.S., F.G.S. 



This paper, being a contribution to the " Fauna of Scotland," 

 published by the Society, is printed separately. 



III. — Notes on the Occurrence of a species of Boring Marine Alga 

 penetrating the Shell Structure of a species of Productus. By 

 Mr. John Young, F.G.S. 



Mr. Young stated that, while preparing specimens of a Productus 

 for microscopic examination of the shell structure, he had occasion 

 to remove the outer surface of the shell with weak acid. After 

 this was done, he found that the inner layer, in many of the 

 specimens, had been burrowed by a minute parasitic organism of a 

 tubular character. These borings on the surface are scarcely 

 visible to the naked eye, but when examined under a low power 

 of the microscope, are seen to branch in certain directions by 

 bifurcation of the tubes as they pass through the inner layers 

 of the shell. In some of the specimens the dark matter filling the 

 tubes is seen to terminate in rounded points : some of the branches 

 also present a moniliform structure of tube; while others, viewed 

 in cross sections on the etched surfaces of the shell, appear as a 

 series of black round dots. Mr. Young stated that at first he was 

 inclined to regard these perforations as the work of a minute bur- 

 rowing sponge, but he had found that a similar organism which had 

 been found perforating the structure of Silurian and Devonian 

 corals and brachiopods, as well as corals from some of the more 

 recent formations, had been described by Professor P. M. Duncan, 

 F.G.S., in the Quarterly Journal of the Geological Society of 

 London, for May, 1876. Professor Duncan, in his paper, regards 

 these perforations as the work of a unicellular alga, parasitic within 

 the structure of the organisms, and clearly related to Achlya; and he 

 distinguishes the form found in the palaeozoic rocks as Palaeachlya 

 perforans. After a close comparison of the Carboniferous organism 

 with that figured by Professor Duncan, Mr. Young w r as inclined to 



