RELATIONS TO EXTINCT ORGANISMS. 



85 



{Monotrypd) nndulata, NIch., present a general resemblance 

 to the forms which constitute the orenus Tetradmni, Dana. In 

 both, the general aspect of the coralhim is at first sight very 

 similar, but there are fundamental differences in their internal 

 structure. Thus, in Tctradmiu the walls of adjoining coral- 

 lites are only doubtfully separate (probably they are really 

 amalgamated, as in ChcEtctcs, Fischer) ; while the corallites, as 

 seen in section, have a peculiarly cruciform or petaloid form 

 (fig. 13, b), due to the presence in each of three, or more com- 



F'g- '3- — A, Fragment of a large corallum of Tetraduim niiiuis, Safford, from the Cincinnati 

 Group of North America, of the natural size ; B, Transverse section of the same, enlarged 

 ten times, showing the petaloid form of the tubes and the short septa; C, Vertical section 

 of tlie same similarly enlarged, showing the tahulie. 



monly four, delicate lamellar septa, which look as if they were 

 formed by inward foldings of the wall, and which extend for 

 a short distance only towards the centre of the visceral cham- 

 ber. The tabulae are usually complete and horizontal (fig. 13, 

 c) ; and even when to some extent incomplete, they never show 

 the peculiar structure which is characteristic of such forms of 

 Moiiticulipora as those constituting the sections Pj'asopora and 

 Pej'onopora. Moreover, the corallites of Tet radium are all of 

 one kind, and there are none of the closely tabulate interstitial 

 tubuli which form such a conspicuous feature in most Monti- 

 culiporoids. 



From all the Helioporidcs, the Monticuliporolds are funda- 

 mentally separated by the total want of pseudo-septa in the 

 large corallites of the colony. In other structural features, 



