MACALISTER ON THE GROWTH OF TURBINATED SHELLS. 19 



1 Habitat, deep water, Irish Sea, occurring among millepora.' Pro- 

 fessor Forbes further states that where the capnea was dredged, the 

 6ea-bottom was chiefly millepora. What is this millepora of the Irish 

 Sea ? It is certainly a deep-water species, a stony coral formed by 

 hydroid animals, and related to the Tabulate madrepores, and is nearly 

 allied to, and, indeed, considered identical with Millepora alcicornn of 

 Linnaeus. 



We are well aware that the growth of these corals which 

 form coral reefs of solid stone is entirely confined to warmer re- 

 gions, their extensions being almost limited to within a few degrees 

 of the tropics. With regard to the depths at which the animals of 

 corals exist, but few seem to have understood the extreme depths in 

 which those hydroids and polyps are in active life. Sir Charles Lyell 

 mentions the several genera of coral that are littoral and sub-littoral, 

 and that the distribution of particular species in regard to the depths 

 of water in which they grow is remarkably uniform. According to 

 Darwin, reef-building corals rarely live at a depth exceeding 120 feet, 

 although it has been shown that some species of coral have been obtained 

 at depths of 900 feet. In temperate climates it is stated that such 

 species as Caryophyllia Smithii are sub-littoral. Dr. A. N. Bell, late 

 Surgeon United States Navy, who has written on Protozoa and on 

 Coral Polyps, says that living coral polyps always work upwards to- 

 wards the light, and that they cannot exist at a greater depth than 

 thirty fathoms. Their peculiar office seems to have been to plant 

 themselves on submarine elevations, and build them up to the surface 

 of the water." 



Dr. Macalister, read the following : — 



Observations on the Mode op Growth of Discoid and Turbinated 



Shells. 



A most interesting paper on the geometrical forms of turbinated and 

 discoid shells was published by the Eev. Canon Moseley in the Philoso- 

 phical Transactions for 1838, p. 351, in which some important points 

 were noticed regarding the geometrical construction of shell -forms. 

 The author of that paper describes discoid shells as generated by the 

 revolution around a central point of the perimeter of a geometrical 

 figure, which latter, although regularly increasing in size, yet remain* 

 always geometrically similar in form. The producing figure in many 

 Gasteropodous Mollusks is represented by the operculum, and in all it 

 may be recognized by making a vertical section in the plane of the 

 radius vector. A turbinated shell is similarly generated, but the gene- 

 rating figure in the production of the helix slips down along the axis 

 instead of revolving in a constant plane. The Eev. Mr. Moseley gives, 

 as illustrations of these { points, measurements of Nautilus pompilius, 

 Turbo pfiasianus, Turbo duplicatus, and Buccinum subidatum, and de- 



