298 TABULATE CORALS. 



surface of the former is studded with Httle quadrangular spines 

 or columns, interspersed in great numbers among the ordinary 

 tubes of the corallum. The structure of these columns was 

 first investigated by me by means of thin sections (Ann. Nat. 

 Hist., sen 4, vol. xviii. p. 93, PI. V., figs. 12 and 12 a); but it 

 is only through more recent and more complete investigations 

 that I have been able to arrive at any definite conception as to 

 their real nature. I am, however, now satisfied that the coral- 

 lum in Dckayia is truly dimorphic, that the surface-columns are 

 the homologues of the spines which are so abundantly de- 

 veloped in M. {Heterotrypd) ttmiida, PhilL, M. i^Heterotrypa) 

 ino7iiliformis, Nich., and other forms of Montiadipora, and 

 that these structures are properly to be regarded as a pecu- 

 liarly modified series of corallites. Taking this view of the 

 subject, the species of Dekayia are principally separable from 

 the spiniferous species of Monticulipora {Heterotrypd) by the 

 fact that in the former the spines are much reduced in number 

 and increased in size, while they are always isolated by the 

 large tubes, these latter being of one kind only. The more 

 minute characters of the sub-genus will appear from the follow- 

 ing very brief description of Dekayia attrita, Nich., the only 

 species with which I am acquainted, and which is very prob- 

 ably identical with the type-species D. aspera, E. and H. 



Dekayia attrita, Nich. 



(PL XV., figs. I - I f.) 



Chatetes attritus, Nicholson, Quart. Journ. Geol. Soc, vol. xxx. p. 503, PI. 

 XXIX., figs. 4, 4ff, 1874; Pal. of Ohio, vol. ii. p. 194, PL XXL, fig. 4, 



1875- 

 Dekayia att?ifa, Nicholson, Ann. Nat. Hist, ser. 4, vol. xviii. p. 93, figs. 12, 

 12 a, 1876. 



Spec. Char. — Corallum dendroid, of subcylindrical branches, 

 varying from four to seven lines in diameter, and dividing at 

 short intervals. Corallites for the most part polygonal, thin- 

 walled, and sub-equal, from eight to ten in the space of one 



