84 PORITES. 



palmata, cdces, flahcUum, and conuda. These forms pass from one to the 

 other by gradations, among which it is impossible to draw the hmit. 

 Tlie flabellum form prevails in rapidly growing specimens, as then the 

 edge is thin and the component small branches more distinct ; but that 

 mode of growth is seen also in the more slowly growing ones, where 

 the branchlets are only more crowded and shorter, and hence more 

 difficult to distinguish. The characters derived from the calicles are 

 affected by the same causes. 



What the Madrepora Thomasiana of Duchassaing and Michelotti may 

 be is not very apparent from their description ; it is probably a variety 

 of the palmed a also. 



This coral is the principal reef-builder, and is seldom found elsewhere 

 than on the outside reef, where it is well exposed to the surf It grows 

 up to low-water mark, but its lower limit I have not ascertained. 



Madrepora cervicornis Lamk. 



This species is found on the reef in rather small isolated bunches ; 

 at Tortugas it is, however, very abundant, forming dense forests in a 

 sort of lagoon in front of Fort Jefferson ; it extends from the bottom 

 to low-water mark in one or two fathoms of water, and requires a 

 rather sheltered position for its full development. I have seen living 

 branches brought up on fish-hooks from seven fathoms. 



Madrepora prolifera La.mk. 



Somewhat less common than the preceding. Some specimens par- 

 take so much of the characters of both this and the preceding species 

 as to shake the belief in their specific difterence. Still, the greater 

 number of specimens examined are readily distinguished, more perhaps 

 by tlieir habitus than by the more minute characters of the calicles. 



Family PORITID^ Dana. 



PORITES M.-Edw. & H. 



Porites Lamk. (pars), 



Porites clavaria Lamk. 

 Very abundant, principally about the reef. 



