VII.J DACTYLOPOEA. 175 



obliquely. In an obliquely transverse section of a tube per- 

 forated 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 Biplopora may have been 

 important factors in the building up of limestone sediments'. 

 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. 18.5) 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^. 



A species of Gyroporella, G. heller ophontis, has recently 

 been described by Rothpletz' from Permian rocks in the 

 Southern Tyrol. The thallns is tubular in form and has a 

 diameter of "5 — 1 mm. 



Dactylopora. 



The genus Dactylopora was founded by Lamarck* on some 

 fossil specimens from the Calcaire Grossier and included among 

 the Zoophytes. D'Orbigny afterwards included it among the 

 Foraminifera, and the structure of the calcareous body has been 

 described by Carpenter' and other writers on the Foraminifera. 



1 Benecke (76) PI. xxiii. ^ Solms-Laubaoh (91) p. 42. 



3 Rothpletz (94) p. 24. * Lamarck (16) p. 188. 



5 Carpenter (62) PI. i. 



