HALYSITID^ AND TETRABIIDM. 227 



allite being united along its whole length with its neighbours to 

 the right and left, and each lamina of the corallum consisting 

 of no more than a single linear series of tubes. Each tube is 

 enclosed in a strong imperforate wall surrounded on its free 

 sides by a thick epitheca, and there is usually a distinct divi- 

 sion of the corallites into two series of different sizes, in which 

 case a single small tube is placed between each pair of the 

 larger tubes. Septa may be obsolete, and, when present, have 

 the form of vertically-disposed rows of spines in cycles of 

 twelve. The tabulse are well developed, complete, not in- 

 fundibuliform nor vesicular, the smaller tubes (when present) 

 being more closely tabulate than the larger ones. 



Obs. — The general form of the corallum in Haly sites is con- 

 stant in all the known species of the genus, and is too well 

 known to need special description. Every corallite in the 

 corallum, except those which form the actual circumference, is 

 united along its whole length, along opposite sides, to two other 

 corallites, those which form the centre of three of the constitu- 

 ent laminae of the mass being similarly united to three of their 

 neighbours. This union is, moreover, not one of mere contact, 

 but is absolute, the epitheca and wall of any given tube being 

 directly continuous with the corresponding structures in the 

 tubes which stand to the right and left of it (PI. XI., fig. i). 

 The most extraordinary feature in the organisation of Haly- 

 sites, however, concerns the mode by which the lateral union of 

 the ordinary corallites of Halysites is effected. If we make a 

 thin transverse section of the form usually known as H. eschar- 

 oides, Lam. (PI. X., fig. 6), we find, as just remarked, that the 

 epitheca and walls of neighbouring tubes are absolutely con- 

 tinuous ; but we observe, further, that the epitheca does not take 

 any part in the formation of the partition which actually divides 

 any tube from its neighbour on either side. On the contrary, 

 the partition in question is formed solely by the coalescent walls 

 of the two contiguous corallites, and it is always of a lighter 

 colour and apparently less compact texture than the rest of the 

 tube. We have, therefore, here the singular fact that each tube 



