GENERA OF FA VOSITID^E. 143 



The structure of these casts was well described by Edwards 

 and Haime (Pol. Foss. des Terr. Pal., p. 209), and the true 

 significance of the most singular of the features presented by 

 the genus was fully recognised by them. They placed the 

 genus, however, in the Perforate Zoantharia, in the immediate 

 neighbourhood of Protarcea and Litharcea, and thus failed to 

 appreciate its relationships with the Favositida. Dr Rominger 

 was, I believe, the first to point out (Amer. Journ. Sci. and 

 Arts, vol. xxxv. p. 82, 1863) that Pleurodictyum was really 

 founded upon casts of a coral allied to Favosites or Mickelinia ; 

 and he has recently united it with the latter genus (Foss. Corals 

 of Michigan, p. 72, 1876). At present, we are acquainted with 

 the actual corallum itself in more than one species of Pleuro- 

 dictyum ; and I shall defer any further remarks upon the genus 

 till I have described the only one of these which I have had 

 the opportunity of examining with proper fulness. 



Pleurodictyum stylophorum, Eaton. 

 (Fig. 22, and PI. VIII., figs, i - i b.) 



Astraa stylophora, Eaton (?), Geological Text-Book, 1832. 



Pleurodictyum Americanum, Ferd. Roemer, Lethaea Palseozoica, PI. XXIII , 



figs. 2 a and 2 b, 1876. 

 Michdinia trochiscus, Rominger, Fossil Corals of Michigan, p. 75, 1876. 



Spec. Char. Corallum discoidal, with a slightly convex 

 upper surface, generally from an inch to an inch and a half in 

 diameter, and about half an inch or rather more in height ; 

 attached by the centre of the base to some foreign body such 

 as the stem of a Crinoid. Lower surface covered by a well- 

 developed epitheca, with strong concentric wrinkles, and also 

 with finer radiating strise. Corallites subpolygonal, very un- 

 equal in size, the larger ones from nearly two lines to about 

 three lines in diameter. Walls thick, perforated by irregularly- 

 placed mural pores. Septa in the form of from twenty-five to 

 thirty-five or more marginal ridges or vertical rows of blunt 

 spines. Tabulae few in number, slightly convex or horizontal, 



