84 PORITKS. 



pahnit!((, dices, Jhibcllitin, ami coniula. These forms pass from one to the 

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

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

 edge is thin and the component small branches more distinct ; l)ut that 

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

 the Ijranchlets 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 Maclrepora Thommidim of Duchassaing and ]\Iichelotti may 

 be is not very apparent from tlieir description ; it is jjrobably a variety 

 of the palmala also. 



This coral is the jirincipal 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, ))ut it,s lower liuiit 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 foi'ests 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. 



Madiepora prolifera Lamk. 



Somewliat less common tlian the ])receding. Some specimens par- 

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

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

 number of specimens examined are readily distinguished, more perhaps 

 by their habitus tlian by the more minute characters of the calicles. 



Family PORITID^ Dana. 



POlilTES M.-Edw. & H. 



Porilcs Lamk. (par^). 



Porites clavaria Lamk. 

 Veiy al)undant. principally about the reef. 



