348 



THE MONTICULIPOROIDS. 



tubes appearing to depend upon their relative age only. On the 

 other hand, most Monticuliporoids exhibit a distinct dimorphism, 

 the colony being composed of at least two kinds of tubes, which 

 differ in size and in internal structure. The relative proportions of 

 these two sets of tubes to one another vary in different forms, but 

 the larger ones may be spoken of as " autopores," and the smaller 

 ones as " mesopores." The " autopores " may be regarded as having 

 lodged the perfect zooids of the colony, and they are distinguished 

 both by their comparatively large size (fig. 225, a), and also by the 

 nature of their tabulae, these structures being usually comparatively 



Fig. 225. — A, Tangential section of Callopora nutnmiformis , Dyb., from the Silurian rocks of 

 Esthonia, enlarged twenty times ; b, Vertical section of the same, similarly enlarged, a, Auto- 

 pore ; m, Mesopore. (Original.) 



few and remote, or being specially modified in form. On the other 

 hand, the " mesopores," or " interstitial tubes," occupy, in greater or 

 less number, the spaces between the autopores, and are not only 

 smaller than the latter, but are more closely tabulate (fig. 225, m). 

 The mesopores have often been considered as merely of the nature 

 of a " ccenenchymal " or " interstitial " tissue, but they may be 

 regarded, with great probability, as really having lodged imperfect 

 zooids, and as corresponding with the " siphonopores " of the Helio- 

 p or idee, and Heliolitidce. 



In addition to the preceding, many Monticuliporoids — whether 

 otherwise homomorphic or heteromorphic — possess yet a further 

 series of tubes, which have been spoken of as " acanthopores " or 

 " spiniform corallites." These are minute cylindrical tubes, with 

 thick and laminated walls (fig. 226), which run in the substance of 



