192 Royal Society. 



It has hitherto been a matter of regret that the Hydroidea were 

 of such a structure as to be unsuitable * for preservation in the 

 fossil state, and that thus we were almost, excepting as far as 

 Graptolites are concerned, without direct cAidence as to the forms 

 which may have been presented by their remote ancestry. We 

 have now two families excellently adapted for preservation as 

 fossils, viz. the IVfilleporidir and the Stylasteridoe. At present no 

 members of these families appear to have been observed in rocks 

 older than the tertiary deposits. A single species only, Dii^tkliopora 

 antiqua, is known to occur in tertiary beds in France, at Chaumont 

 and Yalmondoist; but now that special attention will be directed 

 to these corals, and their structure is better understood, no doubt 

 allied fossil forms \\\\\ be detected. It seems just possible that 

 amongst Pala?ozoic corals such forms as Cijathoncuvia may have 

 been tenanted by a group of hydroid zooids with a large alimen- 

 tarv zooid situate upon the projecting style. CiistlphyUum vesi- 

 culosinn has a crowd of small slit-like pits covering the inner sur- 

 face of its calicle, which have all the appearance of having been 

 tenanted bv hydroid tentacular zooids. I cannot, however, now 

 refer to specimens ; indeed I have never seen any. Ampullae seem 

 to be absent in these corals ; but in shallow-water forms, as in 

 Millepora, they probably would be so. It is quite possible that 

 the Millepores produce Medusce. 



Although the Milleporidae take a very large part in the forma- 

 tion of coral reefs, the iStylasteridae have very little share in the 

 building up of these structures, being for the most part confined 

 to the deep sea. A few species only occur in shallow water, and 

 apparently not in great abundance. In deeper water, howeA'er, 

 the Stylasteridse are most luxuriant. Immense quantities of a 

 large flabellate red Distichopora, brought from the Marquesas group, 

 are sold to tourists at Honolulu. The corals are said to come 

 from deep water. The results of the ' Challenger's ' dredging off 

 the Rio de la Plata in 600 fathoms showed that at that depth 

 very considerable deposits of calcareous matter must be formed by 

 these various genera of hydroid corals, growing associated as they 

 do in masses and attached to one another. Large dead masses of 

 Polypjora brought up by the dredge were especially remarkable, 

 •weighing more than 1 lb., and forming bases of attachment for 

 sponges and all kinds of other animals. 



I am at present engaged in preparing a series of dra^ongs illus- 

 trative of the anatomy of the Stylasteridse, which I hope shortly 

 to lay before the Royal Society, together with a more complete ac- 

 covmt of the structure of these corals. 

 South Atlantic, 



March 24, 1876. 



» Allman, I. c. vol. ii. p. 231. 



t MM. Milne-Edwards & Haime, I. c. t. iii., Appendice, p. 451. 



