GENERA OF FA VOSITID^. 



145 



series of prismatic or subcylindrical columns radiating from a 

 common base, in sucli 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 Pkurodktynm problanaticiim, 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 oi Plcui-odictyiim probleinaticum, 

 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 oi Pkiirodictyum stylophoruni, 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. problcmaticum are, then, 

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



K 



