354 THE MONTICULIPOROIDS. 



7. Lastly, certain extinct forms the Polyzoan nature of which seems 

 unquestionable are hardly distinguishable, as regards minute structure, 

 from other forms which have always been regarded as Monticuliporoids. 

 Thus, an extremely close structural resemblance obtains between Cerio- 

 pora interporosa, on the one hand, and Batostomella {Monticiilipora) 

 tumida, on the other hand. 



As regards their geological distribution, the forms usually regarded 

 as Monticuliporoids are exclusively Palaeozoic, and range from the 

 Ordovician to the Permian inclusive. In the Ordovician rocks, in 

 particular, Monticuliporoids are enormously abundant, and they also 

 constitute a conspicuous feature in the Silurian deposits, while their 

 number is much reduced in the Devonian, Carboniferous, and Per- 

 mian strata. As a rule, particular types are confined to special 

 geological horizons, but the vertical distribution of the group still 

 requires to be worked out in detail. So far as certainly known, the 

 genus Stenopora — a name which has served to cover many different 

 types of the Monticuliporoids — is confined exclusively to the Car- 

 boniferous and Permian deposits. It is not at all improbable that 

 representatives of the Monticuliporoids will be found to have sur- 

 vived into the Secondary period, while there are even Tertiary fossils 

 which show remarkable points of resemblance to this ancient group 

 of organisms. 



With regard to the classification of the Monticuliporoids, two 

 principal types of structure may be recognised, represented respec- 

 tively by the genera Monticiilipora and Fistulipora, as these are now 

 restricted. These two types of structure may be taken with con- 

 venience as the foundation of the two families of the Monticuliporidce 

 and Fistuliporidce. It must be admitted, however, that these two 

 families are connected together by transitional forms ; and it is prob- 

 ably impossible, in the present state of our knowledge, to so define 

 them as to render them mutually exclusive. 



MONTICULIPORID/E. 



The skeleton of the Monticuliporidce is very variable in form, and 

 may be either composed of essentially similar tubes throughout, or 

 may be made up of large tubes (" autopores ") and of a limited 

 number of smaller, closely tabulate tubes (" mesopores "). The 

 interstitial tubes or " mesopores " are not sufficiently developed to 

 isolate the autopores, while they are provided with complete walls 

 and their tabulae are straight. " Acanthopores " are very commonly 

 present. Septal spines are rarely developed. The mode of growth 

 of the skeleton is by intermural gemmation, commonly associated 

 with fission. 



This family includes the genera Mo?iticulipora, Monotrypa, Mono- 



