6 LOWER PENINSULA. 



The figures of the plates could not be numbered without great 

 inconvenience to the printer, on which account I have adopted a 

 rule applying to all, which I think will serve the purpose. 



In all the text references the upper right-hand figure is i ; the 

 upper left-hand figure is 2 ; the lower left-hand figure is 3, and the 

 lower right-hand, 4. Some plates have several figures on each of 

 their two or four principal divisions. In these cases, to save the 

 reader from mistakes and confusion, I have been very explicit in 

 my descriptions, in indicating the particular figure referred to. 



CORALS. 



Corals are sea animals of low organization. The general struc- 

 ture of their body, in the simpler forms, is that of a membranaceous 

 bag, frequently plicated into radially arranged folds. This bag has 

 only one central opening, which serves both as mouth and anus, 

 and is surrounded by a variable number of retractile hollow ten- 

 tacles. In the compound forms the individuals are frequently so 

 intimately united, that the exact demarkation of one body from the 

 other is lost. 



Circulation imperfect, not propelled by a heart. Nervous system 

 very rudimentary ; no special organs of senses. Propagation part- 

 Iv by eggs, forming in the plications within the bag, and ejected at 

 maturity through the central opening, partly by buds sprouting 

 from the surface, or by division and individualization of single parts 

 of the body. Some corals are entirely soft and fleshy ; others se- 

 crete a horny or stony basal skeleton or domjcil, into which the 

 fleshy parts can be partially retracted. All the fossil corals belong 

 to this latter tribe, and this stony Polyparium is the only portion 

 of them preserved. Soft corals, not capable of preservation, have 

 left no traces within the rocks, although, from analogy of present 

 conditions with former, we have a right to assume that they 

 were not missing in the ancient fauna. 



The systematic arrangements under which corals have been de- 

 scribed by various naturalists are very different. 



Milne-Edwards is one of the writers who has paid special atten- 

 tion to the fossil forms of corals. I consider it, therefore, for the 



