IT4 



THE GENUS MONTICULIPORA. 



apart. In spite of this conspicuous difTerence, the more minute 

 external and internal characters of M. ritgosa, E. and H., are 

 precisely similar to those of HI. raviosa, D'Orb. Not only are 

 the characters of the calices and interstitial tubes identical, 

 but no difference of the smallest specific weight can be detected 

 on a comparison of corresponding thin sections of the two 

 forms. This will be rendered evident by a comparison of 



Fig. 19. — A, Tangential section of Monticiilipora rugosa, D'Oib. , enlarged fifty times ; K, Ver- 

 tical section of the same, enlarged eighteen times ; C, Tangential section of M. Dalii, E. 

 and FI. { = M. approximata, Nich.), enlarged fifty times ; D, Vertical section of the same, 

 enlarged eighteen times. All the figures show the two sets of corallites. From the 

 Cincinnati Group, Cincinnati, Ohio. 



tangential and vertical sections of the type-form of HI. 7'aiuosa, 

 D'Orb., Avith similar sections of J/, riigosa, E, and H. In the 

 latter, as in the former, the corallum is composed conspicuously 

 of two series of corallites, the large ones being oval or sub- 

 polygonal, about I -90th to I -80th inch in diameter, and sur- 

 rounded by numerous small tubes (fig. 19, a). The structure 



