A. 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 trabicule 
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. 1c). This mode of arrangement of the 
marginal trabiculee gives also to the septa, when viewed from above (fig. 1a), 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 laminz, an error which has been 
committed by Goldfuss and by ourselves in our first observations. 
Sphenotrochus intermedius is the largest known species of this genus ; sometimes, how- 
ever, S. Milletianus and S. granulatus 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." We have not met 
with any of these young Turbinolide with only six septa and the same number of coste ; 
the youngest in Mr. Searles Wood’s collection (fig. le) has twelve well-marked costz, 
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 costae 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 upper part swells out laterally, and 
the peduncle enlarges and becomes smooth. Soon after this the tertiary coste begin to be 
formed (fig. 1/), 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 ;’ 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 Développement des Polypiers, Ann. des Sc. Nat., 3™° série, 
yol. ix, p. 76, tab. vi. 
2 Loe. cit. 
