MADREPORARIA PERFORATA. 



309 



ber is divided into two halves by two of the six principal septa, which 

 are specially developed, and nearly meet along their inner edges. 



Family 3. Poritidcz. — In this extensive family the corallum is always 

 composite, the constituent corallites being united directly or by the 

 intervention of a more or less copious ccenenchyma. In either case 

 the calcareous tissue of the corallum is spongy or reticulate, and the 

 porous walls of the corallites are mostly not separable from the 



>/.*' V\ ■>'•'■■' ft"* 

 &*»."•-. '-'0V : - •■•."•'; 















Fig. 191. — Madrcpora plantaginea, of the natural size, showing the porous 

 ccenenchyma and the tubular corallites. Recent. 



ccenenchyma, when this latter exists. The septa may be rudimen- 

 tary or obsolete, but they are usually present, and have the form of 

 vertical rows of spinules, which often anastomose, and give rise to a 

 sort of trellis-work. Rarely, the septa are imperforate and lamellar. 

 In some cases tabulae are present, and a columella may or may not 

 be developed. The oldest types of the Poritidce appear in the Ordo- 

 vician rocks, but the family is represented in the Palaeozoic deposits 

 by but a few isolated genera (Protarcea, Arceopora, Palceacis, &c), 



