TUBULirORID^E : -^UBULirORA. 269 



The appearances which this species assumes under different cir- 

 cumstances are so distinct and dissimilar that, without an examina- 

 tion of many specimens, we might readily be led to make two species 

 of them. In what I consider to be its most perfect condition the 

 Polypidom is wart-like, white and calcareous, the surface convex ap- 

 proaching to hemispherical, with the tubular cells radiating from the 

 dimpled centre in rows separated by cellular or reticulated grooves. 

 The tubes near the circumference are closely compacted, those in the 

 centre more loosely, and, from their position, necessarily free or dis- 

 junct at one side, along which there runs a series of spines similar 

 to those that are on the mouth. These pretty specimens have a 

 circular base, and rarely exceed the third of an inch in diameter. 

 They have some pretension to be compared to a madrepora, the 

 elevated rows of tubes representing the radiating lamellae, and the 

 depressed cellular furrows the interlamellar spaces. These consti- 

 tute the most remarkable peculiarity of this Tubulipora, and appear 

 to be formed by a coalescence of the spines of adjacent tubes into a 

 sort of net-work with round or angular meshes, affording in this 

 structure a good demonstration of the organic nature of the poly- 

 pidom. 



In its other most marked state the polypidom forms a circular 

 saucer-like crust, sometimes not much less than an inch in diameter, 

 and bounded by a thin narrow margin. The surface is plain, or 

 only slightly convex, with little elevated roundish spots scattered 

 over it, or waved with transverse ridges. The cells are erect, very 

 close or coalescent, and not distinctly rowed, so that the reticulated 

 spaces become obscure or obsolete, and the spines along the sides 

 cannot be produced. There are three minute denticles on one side of 

 the apertures, of which the central one is largest, and to see them 

 aright it is often necessary to view the specimen obliquely ; but in 

 other specimens growing under shelter, the denticles become elon- 

 gated into strong sharp spines, rendering the surface quite hispid to 

 the naked eye. 



The description of Otho Fabricius is excellent : "Diam. 2 lines. 

 Variat albus, vel flavicans. Tubuli disci per radios plerumque 

 dispositi, versus limbum vero magis aggregati, subcompressi, apice 

 acuminati in aculeos 2 vel 3 divisi superficiem echinatam reddunt. 

 In aliis interstitia radiorum integra ; in aliis, et quidem majoribus, 

 porosa, quasi reticulata. 



" Habitat in plantis marinis et cellulanis variis, quibus planitie 

 sua inferiore adhoeret, frequens satis. 



