Fig. 35, A, affords a diagrammatic view of a _Diplopora_ tube, and shews the arrangement of the numerous whorls of canals. In fig. 35, B, a piece of limestone is represented containing several Diploporas cut across transversely and more or less obliquely. In an obliquely transverse section of a tube perforated by horizontal canals the cavities of the canals necessarily appear as holes or discontinuous canals in the substance of the calcareous wall. The manner of occurrence of the specimens points to the abundance of this genus in the Triassic seas, and suggests that the calcareous tubes of _Diplopora_ may have been important factors in the building up of limestone sediments[323]. In many instances no doubt the carbonate of lime of the thallus has been dissolved and recrystallised, and the original form completely obliterated. As in the rocks built up largely of calcareous Florideae (p. 185) which have lost their structure, it is a legitimate inference that some of the limestone rocks which shew no trace of organic structure may have been in part derived from the calcareous incrustation of various algal genera. _Gyroporella._ Fig. 35, C and D. In this genus from the Alpine Trias the structure of the calcareous tube is very similar to that in _Diplopora_, but in _Gyroporella_ the canals form less distinct whorls and are closed externally by a small plate, as seen in figs. 35, C and D. As Solms-Laubach has pointed out, the branch-systems of _Diplopora_, _Gyroporella_ and other older genera are much simpler than in the Tertiary genera _Dactylopora_ and others[324]. A species of _Gyroporella_, _G. bellerophontis_, has recently been described by Rothpletz[325] from Permian rocks in the Southern Tyrol. The thallus is tubular in form and has a diameter of ·5–1 mm. _Dactylopora._