86 Dr. H. A. Nicholson on the 



Ckcetetes rhomhicus^ Nicholson, Geological Survey of Ohio, 

 Palgeontology, voh ii. p. 201, pi. 21. figs. 12, 12 a. 



Microscopic sections of this beautiful species prove its 

 distinctness from all other previously described members of 

 the group in the most conclusive manner. In cross sections 

 (PI. V. fig. 1) the central portion of the corallum is seen to 

 be occupied by the transversely divided ascending corallites 

 of this region. Each tube is rhomboidal or diamond-shaped; 

 and the corallites are arranged, with geometrical regularity, 

 in a double series of decussating gently curved diagonals. 

 The tubes are filled with transparent calcite ; and each has 

 its rhomboidal area very distinctly and regularly divided 

 into four equal triangles by a cruciform divisional line. 

 These divisional lines in the interior of the tubes are per- 

 fectly regular in their arrangement, and are quite uniform 

 in their direction in each specimen (PL V. fig. 1 a) ; they 

 therefore give rise to a second, fainter, double series of dia- 

 gonal lines, which intersect the more strongly marked series 

 of diagonals formed by the walls of the tubes themselves. 

 Similar, but less conspicuous and less regular, divisional lines 

 are visible in the calcite which fills the tubes of the corallites 

 in many species of Chcetetes and in Constellaria ; but I have 

 been unable to satisfy myself as to the true cause of this phe- 

 nomenon. In^longitudinal sections (PL V. fig. 1 h) the coral- 

 lites are seen to be nearly vertical in the central portion of the 

 ramose corallum, and to curve outwards at a considerable 

 angle as they approach the surface. It is owing to this 

 arrangement of the tubes that the central portion of a cross 

 section (PL V. fig. 1) shows the corallites divided transversely, 

 at right angles to their direction, whilst the marginal portion 

 of a cross section shows the tubes cut obliquely but in the 

 main longitudinally. In the central and vertical portion of 

 their course the corallites are destitute of tabulae ; but these 

 structures are well developed in the outer (more nearly hori- 

 zontal) portion of the tubes. The corallites increase somewhat 

 in diameter in approaching the surface ; and interstitial tubuli 

 are wholly wanting. 



The increase of the corallum is clearly shown by longitudinal 

 sections to take place by fission of the old corallites, and not 

 by gemmation. The species must therefore be placed in the 

 genus Chcetetes^ and not in MonticuUpora as ordinarily under- 

 stood. In fact, so far as my present investigations have gone, 

 all the species ordinarily referred to MonticuUjoora can be 

 shown, by properly prepared sections, to increase by fission of 

 the old tubes ; and they must therefore be placed in Chcetetes^ 



