Prototheca of the Madreporaria. 11 



the falling over is correct (see diagram fig. 4). As the soft parts 

 detach themselves from the base of the prototheca they might 

 be expected to bag down, and they will continue to be acted 

 upon by gravitation and drawn over towards the convex 

 side of the coral until the vertical position has been regained. 

 It is possible that this bearing over to the side may be due to 

 the efforts of the polyp itself to bend up, but gravitation is a 

 causa efficiens. 



In some forms, however, the fossula is not on the dorsal, 

 but on the ventral side. There is abundance of scope for 

 variations of all kinds : a deep cup (that is, the cup of a 

 polyp which grew very slowly in width, for instance) would 

 lie very prone and its fossula would fall over to the dorsal 

 side (diagram fig. 4) ; but a shallower more open proto- 

 theca (that is, one in which the polyp grew very rapidly 

 in width) would, in the prone position, have one (the 

 "ventral") wall nearer the vertical, and this would keep 

 the skin of the point while it hung loose for the while near 

 the ventral side, and the fossula would consequently also 

 appear on this side (diagram fig. 5) *. 



(c) The falling over of the prototheca will explain the 

 departure from a strictly radial symmetry of the septa seen in 

 these curved Palaeozoic corals. It is obvious that, as the 

 coral is bending to the vertical, seen from above, the septa 

 would have the arrangement shown in diagram fig. 6, which is 

 after the classical figure of Kunth showing the septal formula 

 typical of the group called Rugosa. The position of the 

 fossula with relation to this modification of the septal arrange- 

 ment shows that this is the true explanation. Further, it has 

 long been known that, as such corals gradually reacquire a 

 vertical position, the septal arrangement slowly gives up the 

 bilateral and returns to the radial symmetry. Thus the 

 character on which it was proposed to found a great division 

 of the stony corals was nothing but a slight mechanical 



* This is not the first time that this origin of the fossula as a repetition 

 of the tip of the prototheca has heen recognized. Ludwig's figures made 

 it quite clear in 1866 (" Corallen aus palaolithischen Foimationen," 

 Paheontographica, xiv. 1866). But, regarding the skeletons as analogous 

 to the shells of mollusks, to whose shapes he thought they were adapted, 

 he failed entirely to understand the true character of the coral skeleton or 

 of the causes of its changes. 



The use assigned in text-books to the fossula, viz. as a sort of crypt for 

 the sexual products, is probable enough, but need not have been the cause 

 of its origin. I fail to see the evidences for the existence of more than 

 one true fossula in any coral I have examined. Superficial irregularities 

 in the septa, due perhaps to the presence of sexual products, may be quite 

 distinct from the true fossula. A longitudinal section or a fracture 

 showing a complete tabula is the only evidence which can be relied on. 



