39 
We shall now proceed to the examination of aggregated and undi- 
vided madrepores, with distinct stars, and with porous and tuber- 
culous ambulacra. Previously, however, to commencing this investi- 
gation, it is proper to remark, that considerable difficulty frequently 
occurs in the attempt to discriminate between the different madre- 
pores of this class when fossil. This difficulty, which also sometimes 
occurs in the class which has just been treated of, arises from the 
circumstance of the superior surface of the coral, from which its spe- 
cific characters are chiefly taken, being found covered over, and filled 
up, with the extraneous matter, so as to be totally excluded from -the 
sight. In specimens of this kind, and which are by far most numerous, 
no farther knowledge of the form of the coral can be obtained, than 
what is discovered by sections of the specimen ; which being made 
in different directions, yield information respecting the shape and 
size of the stars, and the number and extent of their lamellae. But 
as it is impossible thus to acquire any knowledge of the superior 
surface of the madrepore, so an opportunity is seldom yielded for 
acquiring information of the distance at which the stars are there 
placed ; and never any respecting the peculiar characters of the lamellcB 
or of the amhulacrce. Hence permission must frequently be required, 
in these cases, to substitute analogical inferences for actual observation 
and comparison. 
The figure given by Fougt,^ as madrepora favosa^ represents a fossil 
so much injured and altered, as renders it impossible to determine to 
what species it belongs. 
The specimen figured Plate V. Fig. 9, bears a very strong re- 
semblance, except that its cells are larger, to madrepora retepora. 
The angulated cells are very similar ; but I have not been able, in the 
fossil specimen, to detect the reteporous structure of the parietes ot 
the cells, which gives name to the recent madrepore. This, however. 
* Amoenit. Acad. Tom. I. Tab. IV. Fig- 16. 
