1 48 MADREPOEAKIA, 



Description. — The corallum encrusted flint-pebbles, or shells. If the pebbles were large, 

 it formed thick convex cakes, as much as 12 cm. long by 9 cm. or more across. Sometimes it 

 bound several pebbles together ; if the pebbles were small and isolated so that the colony 

 could not spread laterally, it thickened and formed globular, oval, or dome-shaped stocks, from 



2 to 6 cm. in diameter. 



The calicles are usually but slightly depressed, the skeleton is thin and delicate, the septa 

 are perforated and arranged according to the typical formula, with sometimes a few of the fourth 

 cycle represented ; they never end freely. The tertiaries fuse with the secondaries, and the 

 primaries and secondaries with the columellar tangle, which is always present, sometimes very 

 large, at others only just traceable. 



These are merely the generic characters modified by the accident of having to grow on 

 pebbles. 



There are 32 specimens grouped under this specific name in the British Museum, and the 

 variations can best be described in connection witli the different specimens. All of them are 

 from Bracklesham Bay. 



Specimen No. 1. A large pebble, with six young colonies of different sizes, with wide 

 flattened, epithecal films suiTounding them ; from these films the rudiments of the skeleton 

 can be seen rising. In the smallest or youngest colonies, in which the calicles are also very 

 small, it appears as if multiplication by fission took place ; in the adult gemmation from the 

 reticular wall-angles is the rule, though even here also traces of what looks like fission can be 

 seen. I have found it in no other Goniopora. 



The largest calicles are 3 • 5 mm. and 1 nnn. deep ; in the central regions of the colony 

 they are funnel-sliaped, with sharp, thin, ridge-like walls, with an irregular wavy or zigzag 

 wall-thread ; when sharpest and highest, the thread is straight. The primaries and secondaries 

 start from the ridge, and are thus often conspicuous. The septa are finely serrated, witli frosted 

 or finely echinulate sides. The three cycles are often of different thicknesses. The columellar 

 tangle is not conspicuous. 



This is the specimen referred to and figured by Lonsdale in Dixon's 'Sussex,' 1850, p. 139, 

 PI. I. fig. 5 a. Geol. Dept. 48334. 



These being the characters of the youngest colonies we are perhaps justified in assuming 

 that they are nearest to the ground-form upon which all the other specimens are variations. 

 I accordingly arrange the next seven specimens first in the following list (cf. also Nos. 15 and 

 16), because they show clear traces of having had these characters, with slight variations in 

 sizes of calicles and thickness of septa. Of the more striking variations seen in the remaining 

 specimens some few may be safely referred to abrasion of the surface. Abrasion, for instance, 

 would take off the wall-ridge and show the wall as a reticulum, but it will not account for 

 the different Imids of reticulum which occur. Abrasion would account too for revealing a 

 columellar tangle, in the event of its being obscured in the calicles at tlie surface, but it will 

 not account for the slight development of that tangle in some and its enormous proportions 

 in other specimens. The chief variations met with in the specimens are : — 



The septa are either very smooth and thin, or else very echinulate (cf. figs. 6 and 7). 



They are very perforate, so as to be a mere lattice-work, or else almost entirely laminate. 



They may either run very straight and symmetrical, or tend to be wavy and to lose the 

 radial symmetry. 



