136 PEOE. p. M. DITNCAN^ 0^ THE STRUCTTTEE AlfD 



this so-called genus with Stylina were given, the identity of the 

 two must be conceded. 



The only distinction between the two genera is, that in Heliocoenia 

 the costae do not extend far from the septal margin. The compres- 

 sion of the columella is not invariable, and it is of no physiological 

 importance. In specimens of recognized Stylince, parts may often 

 be found with the characters of Heliocoenia. I agree with the 

 great Prench zoophytologists to a certain extent, and without 

 abolishing the group, I have reduced it to the rank of a subgenus. 

 But no Heliocoenia has a lamellar columella, as drawn by the author 

 of the paper under consideration (plate v. fig. 17). 



Placocoenia, which Mr. Tomes considers to be synonymous with 

 Heliocoenia, is a much better-marked genus than he thinks. He 

 is not justified in saying that d'Orbigny's genus is not sufficiently 

 particularized for adoption. Placocoenia, as illustrated by Goldfuss 

 and described by d'Orbigny, is well characterized as follows : — ■ 

 Colony massive. Calices large, circular or oval, rather distant, 

 united by costae. Columella lamellar and well developed, or in three 

 papillae. Septa entire, unequal, of decameral or of hexameral type. 

 Costae thick and well developed, cristiform or granular. Gemma- 

 tion between the calices. (" Ilevision of the Genera," p. 108.) 



The distinctions between Stylina ( = Heliocoenia) and Placocoenia 

 are obvious. 



Mr. Tomes does not see any alliance whatever between the genus 

 StyloJielia and the Oculinidae, and has no hesitation in removing it 

 into the neighbourhood of Stylina. He gives no reason ; but that 

 given in the "Eevision" can be gleaned by reading the descrip- 

 tions of the recent genera with which it is associated, namely Stylo- 

 jphora and Madracis. It is hard to believe that the figures given 

 by Mr. Tomes on his plate v. figs. 15-17, are from the same speci- 

 men or that they are correct. One calice has a lamellar columella 

 and few septa, and the other has no columella and numerous septa, 

 and is said to be the younger of the two. The arrangement of the 

 costae differs also. 



ISASTE-SIA CONYBEAEII, Ed. & H. 



This species was described and figured by Milne-Edwards and 

 Jules Haime in their moaograph of the British Eossil Corals, Pal. 

 Soc. idL xxii. fig. 4. 



It is a form with large tetragonal calices and a septal number 

 unusually small in relation to the size of the calices. It is charac- 

 terized as follows : — " Calices nearly equal, subtetragonal and cir- 

 cumscribed by a simple edge common to the two adjoining corallites 

 or separated by a slight furrow. Long diameter of the calices 6 or 

 7 lines. ISTo columella." The figure shows the usual sharp edge to 

 the calices, and that the septa have no costal prolongations. The 

 junction of one or two septa in the axis was not considered sufficient 

 to be called a columella. 



This species is now stated by Mr. Tomes not to be an Isastraean 

 but to be the same as a form which was placed in a totally different 



