114 PROCEEDINGS OF THE ACADEMY OP 



in the tubules of any Bryozoon, (Milne Edwards et. H. Arch, du Museum, torn. 

 v. p. 278,) but some Jurassic specimens of Heteropora in my possession ex- 

 hibit with the utmost distinctness their tubules divided by horizontal dia- 

 phragms. It would be difficult to distinguish a vertical section of them, 

 from a similar section of a Chaetetes, if the tube-walls of the first were not 

 perforated by densely crowded, very minute pores, while the walls of a Chae- 

 tetes are imperforate. 



Fisher, the author of the genus, informs us that the tubes of Chaetetee 

 multiply by division, while other observers, in specimens believed to be 

 Chaetetes, could only see a multiplication of tubes by lateral gemmation, and 

 therefore, to avoid the difficulty, created the genera Stenopora and Monticu- 

 lipora, for these specimens. Milne Edwards is, to my knowledge, the only one 

 to affirm Fisher's observation to be true, (British Fossil Corals, p. 264,) but he 

 does not specially designate the species on which he made his observations, 

 and subsequently places all the species he formerly named Chaetetes, under 

 the genus Monticulipora. 



I know of only one fossil resembling Chaetetes, in which the tubes are 

 multiplied by division ; this is the genus Tctradium, whose tubes regularly di- 

 vide into four parts, but there is no reason to suppose this to have been the 

 type for Fisher's genus Chaetetes, nor seems it probable that Milne Edwards 

 had it under consideration. The structure of Chaetetes is considered to be 

 exclusively tubular. 



If we observe the different forms of Chaetetes, we will find some with con- 

 tiguous polygonal orifices, and thin intervening walls. Others we will see 

 with the tube mouths rounded, only partially contiguous, and with a num- 

 ber of smaller angular openings dispersed between them. In still others, 

 the orifices are circular, not in contiguity, and surrounded on all sides by 

 smaller angular openings. A vertical section through these different kinds 

 will, at first sight, not exhibit a corresponding variety of appearance ; we 

 find the whole corallum to be an aggregation of tubules, which are divided 

 by transverse diaphragms ; a closer examination, however, will reveal to us, 

 in the last mentioned forms, two sorts of tubules : larger ones, more or less 

 circular in the cross-section, with straight diaphragms at variable, sometimes 

 quite remote distances ; and smaller ones, which are angular, with more closely 

 approximated diaphragms ; but the different tube segments, cut off by the di- 

 iphragms, are not always so regular as the nature of a tube would require it ; 

 some are projecting over the others, and joining with the adjacent segments 

 in zigzag lines, which is a sure evidence that we have no real tubules before 

 -us, but merely vertical rows of independent cells, which being crowded in 

 between tubes, assumed themselves the shape of tubules. 



An interesting family-mark, common to Chaetetes, and to a number of 

 other genera related to it, are the peculiar maculae noticeable on their sur- 

 face. In specimens of prevalently tubular structure, these maculae are con- 

 stituted by aggregations of larger tubes than the others ; at the same time we 

 see the surface at these places frequently elevated into small monticules. In 

 other specimens, where the intertubular cell-mass is well developed, these 

 maculae are contrasting with the other surface by their entirely cellulose 

 structure, and it is not uncommon to see these spots depressed, instead of 

 being elevated. 



The orifices of Chaetetes are generally open, or exhibit some distance be- 

 low the surface their diaphragms, which appear to be perfect. It is, however, 

 not rare to find specimens in which the tubules are closed by opercula with a 

 central opening. In specimens of Chaetetes rugosus and ramosus, from the 

 blue limestone of Cincinnati, a part of the surface frequently has closed tub- 

 ules ; their appearance assumes hereby an entirely different character, which 

 reminds one greatly of the ramulets of Melicertites from the Oolite forma- 

 .tiou. Also .of Chaetetes frondosus, I have some specimens exhibiting opercula. 



[May, 



