SUB-GENUS MONOTRYPA. 169 



{loc. cit. siipni). More particularly, I find that it is necessary 

 to admit into this section forms in which the walls may be 

 more or less thickened, though the thickening is never of such 

 a nature as to lead to an apparent fusion of the walls of adjoin- 

 ing corallites. The presence of clusters of tubes somewhat 

 laroer than the avera2:e is also not a constant feature. The 

 chief distinctive character of Monot^ypa is that there are no 

 closely tabulate interstitial tubes, such as exist in all the other 

 sections of Monticulipora. There is thus no direct evidence 

 that the corallum was dimorphic. The large tubes of the 

 clusters may have lodged specialised zooids ; but this is doubt- 

 ful, since they are not always present, and, in any case, their 

 internal structure is precisely the same as that of the average 

 corallites. It is possible, also, that two sets of zooids were 

 really present, but that the corallites which these inhabited 

 are not distinguishable by any marked structural feature. 

 When present (as in ]\I. discoidca, James), the spiniform coral- 

 lites may be taken as a sign of dimorphism ; but these struc- 

 tures are usually wanting in Monotrypa, and their presence 

 does not alter the fact that the normally present, closely tabu- 

 late interstitial tubes are uniformly wanting. T\\^foriii of the 

 corallum in the Monticuliporoids of this section is extremely 

 variable, being massive, discoidal, ramose, laminar, or encrust- 

 ing, in different types. From Ckcstetes, Fischer, Moiiohypa 

 is sufficiently distinguished, among other characters, by the 

 fact that the corallites never become truly united by their walls. 

 From the other sections of ]\Ionticulipo7^a, as from Fistulipora, 

 M'Coy, the species of Alonotrypa are sufficiently separated by 

 the want of a proper series of interstitial tubes. 



Dr Steinmann (N. Jahrb. fiir Min., &c., 1880, p. 438) is dis- 

 posed to think that the section Monotrypa cannot be maintained 

 because the form which I have named M. Wiuteri is really 

 a Favosites. I am not, however, able to admit (as elsewhere 

 more fully shown) that there are any good grounds at present 

 for regarding ]\L Wiiitei^i, NIch., as a Favosites; and even if 

 there were, it would not affect the validity of the section Mono- 



