DESCRIPTIONS 



OF Genera and Species. 



Of the considerable number of genera liere discuHsed, several are repre- 

 sented by many species whose differentials may seem obscure or, to some 

 students, overestimated. In fixing specific values within a given genus of such 

 variable organisms, it has been found necessary to depend very largely upon 

 variations in form as the paramount specific character. If, in this practise, 

 what may seem, in such prolific genera as Prismodicita, Hydnoceras and 

 Thysanodictya, to be traits of lesser value, are dignified with specific distinc- 

 tion, it is because such distinctions are, in the existing state of our knowledge, 

 the truest expression of these relations, as well as the most convenient 

 denomination. To future investigations with the aid of more extensive 

 material, must be assigned the detennination of fundamental type forms, in 

 terms of which all of these manifold variations may ha expressed. 



SPECIES OK THE SILURIAN. 



DICTYOSPONGIA, gen. nov. 

 (For description see page 72.) 



DicTYOsPONGiA Danbyi, McCoy (sp.). 



1855. Tetragonis Danbyi, McCoy. British Palaeozoic Fossils, p. 62, pL 1 D, 



figs. 7, 8. 

 1880. Tetragonis Danhyi, F. Roemer. Lethaea Palaeozoica, Part 1, p. 304. 

 1883. Dlclyophijton Danbyi, Ilinde. Cat. Fossil Sponges British Museum, 



p. 131. 



1887. Dictyophyton Danbyi, Hinde. Monogr. British Fossil Sponges, Part 



1, pi. ii, figs. 4 a-c. 



1888. Dictyophyton Danbyi, Ilinde. Monogr, British Fossil Sponges, Part 



2, p. 128. 



" Sponges subovate or sub-conical in form, growing from an obtuse basal 

 point, without stem, root or point of attachment; the base is flattened or 

 convex, the greatest width in some specimens is just above the base, in others 

 about half the height of the sponge, from this it gi-adually tapers to the 



