GENERA OF CH^TETIDM AND MONTI CULIPORIDJl. 327 



tions (fig. 43, D, b) show also that their internal structure dif- 

 fers entirely from that of the large tubes, and resembles that 

 of the smaller corallites among the Monticuliporidce generally, 

 the visceral chamber being simply crossed by crowded, com- 

 plete, and horizontal tabulae. 



As regards its affinities, there cannot be any doubt but that 

 Prasopora is a genuine Monticuliporoid, and that it is most 

 nearly allied to that section of Montimlipora which I have 

 named Diplotrypa, and of which M. petropolitana, Pander, is 

 the type. There is, indeed, a distant approach to the peculiar 

 structure of the large corallites of Prasopora in the large vesi- 

 cular tabulae which are commonly found on one side of the 

 visceral chamber in the corresponding corallites of M. {Diplo- 

 trypa) Whiteavesii, Nich. (see PI. XIV., fig. i). The existence, 

 however, of a complete peripheral zone of vesicular tabula;, 

 enclosing a central tube, is a feature of an entirely unique 

 nature, and fully entitles Prasopora to take rank as a distinct 

 genus. 



The only known species of Prasopora occurs commonly in 

 the " Craighead Limestone " (Lower Silurian) of Craighead, 

 near Girvan, Ayrshire, whence it was collected by Mrs Robert 

 Gray. 



Genus Dania, Milne-Edwards and Haime, 1849. 

 (Comptes Rend., t. xxix. p. 261.) 



The genus Dania was proposed by Milne- Edwards and 

 Haime for the reception of a single species {D. Huronica) 

 from the Upper Silurian of Drummond Island, Lake Huron ; 

 and they give the following as the characters of the genus (Pol. 

 Foss. des Terr. Pal., p. 275) : — 



" Corallum massive, composed of prismatic corallites, which 

 are intimately united by their well-developed walls. Tabulae 

 completely horizontal, and continued across contiguous cor- 

 rallites in such a manner as to give rise to continuous lamella?, 



