246 TABULATE CORALS. 



by a concentrically-striated epitheca, while the upper surface 

 carries the calices. Corallites in complete contact throughout, 

 divisible into two distinct series. The larger tubes are com- 

 paratively few in number, and possess twelve lamellar septa, 

 formed by infoldings of the wall, and extending but a short 

 distance into the visceral chamber ; while they are crossed by 

 horizontal, complete, and comparatively remote tabulae. The 

 smaller tubes are devoid of septa, are comparatively irregular 

 in form, and only possess distinctly differentiated walls when 

 young. In their adult state their walls become amalgamated 

 with the convex and anastomosing tabulse by which their 

 cavities are intersected, in such a manner that they can no 

 longer be recognised as distinct structures, and the interspaces 

 between the larger corallites become filled up with a loose 

 tissue composed of irregular lenticular vesicles. 



Obs. — In almost all the essential features of its anatomy, 

 Plasmopora entirely resembles Hcliolites, and it is only neces- 

 sary here to make a few remarks on the sole character by which 

 the two genera can be separated — namely, the peculiar struc- 

 ture of the smaller tubes. In Heliolitcs, as we have seen, the 

 smaller corallites are polygonal in shape, have perfectly distinct 

 walls, and are crossed by essentially horizontal tabulae, which, 

 though often placed at the same level in neighbouring tubes, 

 do not actually coalesce with one another laterally. In very 

 young s^&cim&ns oi Piasmopoi^a petaliformis, E. and H., — the 

 only species of the genus that I have examined by means of 

 thin sections — the condition of parts is so far like that of 

 Heliolites that the walls of the smaller corallites are perfectly 

 recognisable (PI. XII., fig. i) in long sections ; but there is this 

 difference, that the tabulae are now very highly convex, and are 

 either continued into one another, or are joined with the walls 

 of the tubes in such a manner as to give rise to an apparently 

 continuous vesicular tissue, which fills all the spaces between 

 the larger corallites. In older coralla this amalgamation of the 

 curved and inosculating tabulae with the walls of the tubes has 

 gone so far, that the latter almost or quite disappear. Hence, 



