.? UBDI ] 7 S IONS. 9 9 



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., ser. 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 Dekayia is truly dimorphic, that the surface-columns 

 are the homologues of the spines which are so abundantly 

 developed in M. {^Hctcrotrypa) hiinida, Phill., AI. {Hderotrypa) 

 monilifoi'mis, Nich., and other forms of Moiiticiilipora, 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 by the fact that in 

 the former the spines are much reduced in number and in- 

 creased in size, while they are always isolated by the large 

 tubes, these latter being of one kind only. It cannot be denied, 

 however, that the separation of Dekayia from Alonticiilipora, 

 so far as our present knowledge goes, is purely arbitrary, and 

 is only defensible upon the ground that its surface-columns 

 constitute a marked external character, by which its species 

 can be readily and conveniently distinguished as a group 

 apart. 



The known species of Dekayia are confined to the Silurian 

 rocks. 



(IV.) Monticulipora, D'Orbigny, 1850. 



In the restricted sense in which the name Monticulipora is 

 here used (that is to say, with the exclusion of Fistulip07^a, 

 Coiistellaria, and Dekayia), the genus may be defined as in- 

 cluding coralla composed of tubular corallites, which are mostly, 

 though not always, of two distinct kinds, differing from one 

 another in relative size, and also in their tabulation. The 



