104 PKOP. p. MAETIN DUNCAN ON THE 



calices about one half of the diameter of the largest. These septa 

 reach the columella. Usually, but not invariably, there is a very 

 small septum between two of the larger. Still smaller corallites 

 have six primary septa and six very small secondary ones (fig. 7). 



In no instance is there a second or third small septum between 

 two larger and primary septa. The endotheca is scanty and the 

 " dissepiments stretch from one septum to its neighbour here and there, 

 but the calibre is never partitioned off. 



As, on account of the close contact of the walls of corallites, gem- 

 mation can only take place on the walls and from within the calice 

 close to the margin, it is not readily discovered in sections. The 

 best examples show a considerable increase in the diameter of the 

 corallite and the formation of a bud with six or seven large and 

 some small septa, the septa and columella of the parents being 

 somewhat confused and elongated (fig. 8). 



There are many broken corallite- ends to be seen on the upper 

 part of the mass, and their being continued from those already 

 described is evident. But with approach to the surface the thick- 

 ness of the united walls usually diminishes. 



In one little spot some perfect calices have been preserved, and, 

 although weathered, they show most unmistakably the Astrocoe- 

 nian character. The united walls are narrow and give indications, in 

 every instance, of a polygonal outline. There are varying amounts 

 of enlargement and swelling of the top of the walls, over which the 

 costse cross after the manner of those of other true Astrocoenice *. 

 There is no separation of the corallite-walls, and there is no instance 

 of gemmation to be seen there. 



The calices are shallow, and the columella projects slightly, and 

 there is no crateriform and conical elevated margin. 



There are some very good specimens of Astrocoenia gihhosa in the 

 Museum of Practical Geology in Jermyn Street, and in one the 

 variability of the size of the calices and of the thickness of the 

 united walls at the surface is considerable. The polygonal shape of 

 the terminations of the corallites is almost invariable, and when a 

 lumen appears to be circular or nearly so, it is evident that it is 

 included in a polygonal corallite. 



The intercalicular structures, that is the united walls, are raised 

 very generally at the corners of the calices, and the local swelling is 

 very decided and gives a very uneven appearance to the surface. 

 The usual dimensions of the calices is (lumen) 1 mm. ; and the width 

 of the united walls may be one half or one third of a millimetre. 



In another specimen the variability of the size of the calices is 

 striking ; but they are pentagonal in outline and not circular. 

 The united walls are thinner than in the other specimen, and are 

 usually about one fourth of the diameter of the lumen of a calice. 

 They are raised into decided elevations at the corners of the 

 calices. 



* No Astrocoenian has all the septa of one cahce blending with those of an- 

 other (Quart. Journ. Geol. Soc. vol. xxxix. p. 558), and no Jurassic species has 

 four cycles of septa. 



