314 THE PROCEEDINGS OF THE LINNEAN SOCIETY 



This genus is allied to Ceratotrochus and Platytrochus. It differs 

 from the former in its rudimentary columella and simple small 

 septa ; from the latter in its simple costee and septa, and from both 

 in the extraordinary depth and width of the calicular fossa, 

 whence the name from KoiXas, a cavity. 



There is a fossil species from Muddy Creek named by me in 

 the Proc. Boy. Soc, p. 1877, Smilotrochus vacuus, which I think 

 should be referred to this genus. 



Genus Placotrochus, Edw. & H., 1848. 



Corallum simple, straight, compressed, pedicellate with a basilar 

 scar : columella lamellar with a horizontal and crenulate edgre ; 

 septa exsert. 



Placotrochus candeanus, D'Orbigny. 



Corallum elongate, with almost parallel lateral costas, with ru- 

 diments of crests on the inferior half and close to the broad 

 basilar scar two small compressed spines ; calice regularly elliptical 

 with almost horizontal edges ; fossa not very deep ; columella 

 very extended ; septa thin with frilled edges and very granular, in 

 four cycles ; first and second equal, third differing slightly. N.E. 

 Australia, Princess Charlotte Bay, also found in the China seas. 

 I believe that it extends outside the tropics. 



Placotrochus elongatus, Duncan, Quart. Jour. Oeol. Soc, 1870, 



p. 300,^. 20, fig. 3. 



The following are the notes of Professor Duncan on this 

 species: — "The coral is very tall in relation to its breadth, straight, 

 greatly compressed, especially inferiorly, finely pedicellate and 

 cuneiform. The sides are rounded and slightly swollen out here 

 and there, and form an angle of about 15 to 20, or are sharp and 

 slightly spined, but the spines do not project much beyond the 

 epitheca (in old specimens) ; the anterior and posterior surfaces 

 are flat ; the calice is small elliptical and rounded at the sides ; 

 it has slightly exsert septa, which are rounded, thin, delicate, and 

 unequal, and in six systems of four cycles ; the fossa is central, 



