GENERA OF FA VOSITID&. 145 



series of prismatic or subcylindrical columns radiating from a 

 common base, in such a manner that those nearest the peri- 

 phery are horizontal or even reclined, and those in the centre 

 are vertical, while intermediate columns have correspondingly 

 intermediate grades of inclination. These columns are some- 

 times markedly striated with longitudinal ridges, as well as 

 marked with small rounded elevations (fig. 22, B) ; while each is 



Fig. 22. A, Lower surface of the cast of Pleurodictyum problematicum, Goldf., from the Lower 

 Devonian of Germany, of the natural size (after Roemer), showing the vermiform body in 

 the centre ; B, A few of the separate casts of the tubes of Pleurodictyum problematicum, 

 Goldf. , from the Devonian of the Eifel, showing the casts of the mural pores and inter- 

 septal grooves, enlarged (after Milne-Edwards and Haime) ; c, Upper surface of the coral- 

 lum of Pleurodictyum stylophorum, Eaton, from the Hamilton group of North America, of 

 the natural size, showing the form of the calices (original) ; D, Lower surface of another 

 example of the same, of the natural size, showing the striated epitheca, and the point 

 where the corallum was attached to the stem of a Crinoid (original). 



connected with its neighbours by a number ot cylindrical rods. 

 The above appearances are now readily intelligible, as, indeed, 

 they were to all intents and purposes even to the authors of the 

 ' Polypiers fossiles,' though these distinguished observers had 

 never seen any actual coral capable of giving rise to a similar 

 cast. The prismatic columns of P. problematicum are, then, 

 casts of the polygonal corallites of a Favositoid corallum, of a 



K 



