370 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



Pourtales.says of the species: 



The same remarks about variation, given under the head of 0. cavernosa, can be 

 applied to this species; there are very fine examples in the museum of the great 

 A-ariation of form of the caHcles in the same specimen. 



It is very common in Florida on the reef and in the channels, and forms large 

 hemispherical masses nearly up to low- water mark. The central and highest part 

 often dies out from being left uncovered at very low tide and the mass then assumes 

 an annular form through the decay of the dead part.^ 



Verrill writes: 



It shows considerable variation in the size of the calicles; in the extent to which 



they are crowded together; in the prominence of their borders above the intervening 



exotheca; in the prominence of the septa above the walls; and in the extent to which 



the small septa of the third cycle are developed. But yet these variations, so far as 



I have seen, never go so far as to render difiicult the recognition of the species unless 



the specimens are badly worn. 



******* 



When well grown it forms hemispherical or spheroidal masses, up to 5 feet or more 

 in diameter. But it also gi'ows in ii'regular incrusting plates, and sometimes in nodose 

 or lobulate masses, or even in branched forms. - 



Duerden, in describing specimens from Jamaica, says: 



The species occurs on coral areas in small or large, fixed, nearly spheroidal masses; 

 also as an incrustation occupying areas several feet across. Small isolated colonies 

 are sometimes conical. In places it is an important constituent of the reefs.^ 



This is one of the species to which I devoted a great deal 

 attention in my study of the living reefs in Florida and the Bahamas, 

 and have inserted references to two of my papers (1915, 1916) in which 

 it is considered. It is preeminently the great reef-building species 

 of the Pleistocene and Recent reefs in Florida and the West Indies. 

 Where there is sand on the bottom, it forms tall, thick, round-topped 

 columns. 



VARIATION OF FOSSIL SPECIMENS. 



There are specimens, particularly those of known Pleistocene age, 

 similar to the typical form of the species, except that there may be 

 variations in the size of the calices; those of a specimen from Fort 

 Nassau, Curasao, range from 3 to 4.5 mm. in diameter, measured 

 between thecal summits; those of another specimen from West- 

 punt, Curarao, are from 2.5 to 3 mm. in diameter. The former- 

 possesses the largest calices of any specimen of the species I have seen. 



The variations not included in the preceding remarks may be 

 divided into two classes, dependent upon growth — namely, a, ex- 

 planate or incrusting; &, columnar. 



A. Growth from explanate or incrusting. 



1 Mus. Comp. Zool. 111. Cat. No. i, p. 72, 1871. 



- Conn. Acad. Arts and Sci. Trans., vol. 11, pp. 95, 96, 1902. 



3 West Indian Madreporarian Polyps, Nat. Acad. Sci. Mem., vol. 8, p. 56i, 1903. 



