4 BRITISH FOSSIL CORALS. 



not quite as much developed as the older ones, but are broad enough to reach the columella, 

 to which they are united, at least towards their base. The union between the septa and 

 the columella is not complete, but is effected by means of a double series of trabiculse 

 extending from the inner edge of the septa, bent alternately to the right and to the left ; 

 so that in a vertical section of the visceral chamber a series of pores is seen along the line 

 of junction of each septa with the columella (fig. lc). This mode of arrangement of the 

 marginal trabiculse gives also to the septa, when viewed from above (fig. la), or by means 

 of a horizontal section, the appearance of bifurcation along their inner margin, and may 

 easily be mistaken for a disjunction of their two constituent laminae, an error which has been 

 committed by Goldfuss and by ourselves in our first observations. 



SphenotrocJms intermedins is the largest known species of this genus ; sometimes, how- 

 ever, S. Milletianus and S. gramdatus are almost as long. Its usual length is about three 

 lines, but there are individuals half an inch long. The long axis of the calice is about two 

 lines and a half. 



Mr. Searles Wood, to whose kindness we are indebted for the specimens here described, 

 has collected an interesting series of these Corals, showing the changes of form which they 

 experience before arriving at the adult state, and has thus enabled us to study their mode 

 of growth, as we had already done for Fungia in a preceding memoir. 1 We have not met 

 with any of these young Turbinolidse with only six septa and the same number of costse , 

 the youngest in Mr. Searles Wood's collection (fig. \e) has twelve well-marked costsp, 

 distinct from the top to the bottom of the corallum ; but the six primary septa are the 

 only ones which are pretty well developed, and those of the second cyclum are still in a 

 rudimentary state. There is no trace of the columella, which appears at a later period 

 and the general form of the corallum is almost cylindrical ; its height is then not more 

 than two thirds of a line, and its calice is circular. The base of the corallum is 

 circular ; it is truncate, but not spread out, and its adherence must have been of very short 

 duration. 



Before the tertiary costse make their appearance, the calice begins to enlarge in one 

 direction more than in the other, so as to assume an oval form ; a slight coarctation 

 becomes visible towards the middle of the corallum, its ivpper part swells out laterally, and 

 the peduncle enlarges and becomes smooth. Soon after this the tertiary costae begin to be 

 formed (fig. If), and the calice becomes completely elliptical, but is still quite horizontal. 

 The coarctation above the peduncle still exists, and we at first supposed that the upper 

 part of the corallum became free by rupture, as is the case in Flabellum ; 2 but the series 

 of specimens collected by Mr. Searles Wood shows that such is not the case, and that the 

 peduncle does not lose its vitality, but is gradually absorbed. Its truncate extremity is first 



1 Observations sur la Structure et le Mode de Developpement des Polypiers, Ann. des Sc. Nat., 3 ne eerie, 

 vol. ix, p. 76, tab. vi. 



2 Loc. cit. 



