THECID^ AND HELIOPORID^. 245 



smaller corallites (PI. XII., fig. 2 a), and in thin sections appear 

 to be of a much darker colour than the actual walls of the 

 tubes. In the small corallites the tabulae are numerous, com- 

 plete, more or less horizontal, and often placed at corresponding 

 levels in contiguous tubes. In the larger corallites the tabulae 

 are also complete, and essentially horizontal, but they are 

 placed at somewhat greater distances apart, and they occasion- 

 ally unite with one another. 



I have made an examination by means of thin sections of 

 H. interstincta, Linn., H. Mttrchiso7ii, E. and H., H. megastoma, 

 M'Coy, H. porosa, E. and H., and H. Gi^ayi, E. and H. ; 

 but none of these depart in any noticeable feature from the 

 normal type of the genus, or exhibit peculiarities of such 

 importance as to justify special description. The most aber- 

 rant species of the genus which I have examined is a form from 

 the Devonian of Australia, which will be subsequently de- 

 scribed by Mr R. Etheridge, jun., and myself under the name 

 of H. plasmoporoides, and which nearly approaches the genus 

 Plasnwpora by its comparatively irregular and comparatively 

 few interstitial tubes. It shows itself, however, to be a true 

 Heliolites by the complete walls of the smaller corallites, and 

 by the fact that the tabulae of these tubes are not curved or 

 vesicular. Another curious type is the Upper Silurian H. 

 dubia, Fr. Schmidt, but I have had no opportunity of investi- 

 gating this species microscopically. 



So far as at present known, all the species of Heliolites are 

 confined to the Lower and Upper Silurian rocks, and to the 

 Devonian. 



Genus Plasmopora, Edwards and Haime, 1849. 



(Compt. Rend., t. xxix. p. 262.) 



(PI. XL, fig. 5, and PI. XII., fig. i.) 



Gen. Char. — Corallum discoidal when young, but becoming 

 hemisiDherical or spheroidal when adult, its lower surface covered 



