On a new Genus of PalcBozoic Fossils. 463 



XLVI. — On Ascodictyon, a new Provisional and Anomalous 

 Genus of Palceozoic Fossils. By H. Alleyne Nicholson, 

 M.D., D.Sc, F.R.S.E., and R. Etheridge, Jun., F.G.S. 



[Plate XIX.] 



The curious little fossils for which we propose the generic 

 title of Ascodictyon are parasitic in their habits, and are found 

 adhering to the shells of Brachiopods, the exterior of corals, 

 or the stems of Crinoids. We are acquainted with at least 

 three distinct forms, one of which occurs in the Carboniferous 

 rocks of Scotland, whilst the other two have been detected in 

 the Devonian deposits of North America. In all the members 

 of this group the organism, though visible to the naked eye, 

 can only be properly examined by means of the microscope, 

 and consists of minute calcareous vesicles, the walls of which 

 are more or less extensively perforated by microscopic fora- 

 mina. The vesicles or " cells," whatever their shape or ar- 

 rangement may be, are always hollow ; but they exhibit no 

 definite aperture, save the very minute pores just spoken of. 

 In some cases they open into one another by short contracted 

 necks or stolons, thus forming a loosely reticulate network ; 

 whilst more typically they are arranged in regular, usually 

 stellate clusters, which in turn are united with one another by 

 delicate thread-like hollow tubes, which often ramify and ana- 

 stomose. 



The above being the general characters of Ascodictyon., a 

 provisional generic diagnosis may be framed as follows : — 



Gen. char. Organism composite, parasitic, adherent on 

 foreign bodies, composed of numerous calcareous cells or vesi- 

 cles, the walls of which are perforated by a greater or less 

 number of microscopic foramina, but which possess no single 

 large aperture. The cells may be united almost directly by 

 the intervention of short tubular necks ; or they may be dis- 

 posed in clusters connected with one another by hollow fila- 

 mentous tubes, which usually anastomose, and which in 

 some cases, at any rate, are likewise perforated by microscopic 

 pores. 



As before remarked, the genus, so far as our present know- 

 ledge goes, is confined to the Devonian and Carboniferous 

 periods; and the following are the characters of the three species 

 with which we are as yet acquainted. 



Ascodictyon fusiforme^ Nich. and Eth., Jun. 

 (PI. XIX. figs. 7, 8.) 



/Sjpec. char. Colony composed of fusiform, sometimes pyri- 



