282 ZOANTHARIA. 



while the " counter " septum is especially long. On the other hand, the 

 fossula may be placed on the concave (or " ventral ") side of the simple 

 coralla, as in some of the species generally placed in Zaphrentis {e.g., 

 in Zaphrentis Enniskilleni, fig. 139). In this case the fossula is inter- 

 sected by the "counter" septum, which is reduced in size, or may be 

 altogether rudimentary or obsolete ; while the " cardinal " septum may 

 have its normal dimensions, or may be specially developed. In rare 

 cases the fossula (when single) is lateral in position, and is occupied by 

 one of the " alar" septa. 



The interseptal loculi in the Rugose corals are generally more or 

 less largely occupied by endothecal tissue in the form of dissepi- 

 ments and tabulse, both these structures being usually conjoined. 

 The dissepiments are principally developed in the peripheral area 



¥N/^r TZ^^WTT^I 





'T/v% 









Vvt \s- 















— "~ 



M/TJ 





















^S\Cl 



g A B 



Fig. 159. — Structure of Omphyma subturbinata, from the Silurian of Britain. A, Transverse 

 section, enlarged slightly, showing the four fossulae, the arrangement of the septa, and the large 

 central area occupied by the tabulse : h, Cardinal septum ; ss, Alar septa ; g, Counter septum, b, 

 Longitudinal section, slightly enlarged: e, Peripheral zone of vesicular dissepimental tissue; t, 

 Tabulse. (Original.) 



of the visceral chamber, and have the form of curved calcareous 

 plates directed obliquely from one septum to the next one. In 

 transverse sections (fig. 159, a) the cut edges of the dissepiments 

 appear as transverse lines passing directly or in a curved manner 

 across the interseptal loculi, and connecting together adjacent 

 septa. In longitudinal sections (fig. 159, b) the dissepiments are 

 usually seen to give rise to an outer zone of lenticular vesicles (e) 

 which are directed obliquely inwards and downwards from the in- 

 ternal surface of the wall towards the centre of the visceral chamber. 

 In the central region of the corallum the dissepiments become more 

 nearly horizontal and are developed in adjacent interseptal loculi at 

 the same level, their coalescence giving rise to the flat or curved 

 transverse partitions which are known as "tabulse" (fig. 159, b, /). 

 The central tabulate area of the visceral chamber varies much in 

 extent, but is best developed in those types (such as Amplexus i 



