116 PROCEEDINGS OP THE ACADEMY OP 



ter explain than by comparing it with certain decorations of watch cases, con- 

 sisting of concentric circle lines crossing each other. Chaetetes pavonia, with 

 the synonytne Ptylodictya pavonia D'Orbigny, is described by Milne Edwards 

 amongst the Chaetetes forms of the Cincinnati limestone. 



This species has indeed a great resemblance to the group to which Ptylo- 

 dictya belongs. It grows in double, thin laminae, separable in two folia, 

 which have on the inner side a dermatic concentrically wrinkled and striated 

 crust, exactly similar to the separated leaves of Ptylodictya. The tubes be- 

 gin with prostrate, thin walled ends, and become rectangular to the surface, 

 by abruptly bending upwards ; the erect part of them exhibits very thick 

 walls. The orifices are contiguous, slightly dilated, and arranged in undu- 

 lating rows, which, crossing each other under oblique angles, make their 

 outlines more or less regularly rhomboidal. The outlines of the single tubes, 

 however, are polygonal, and may be plainly distinguished in the centre of the 

 massive interstitial spaces. Diameter of tubes one-sixth of a millimeter, 

 somewhat larger on the monticules, which are little elevated and are dissemi- 

 nated over the surface at a distance of three or four millimeters. No dia- 

 phragms observed. Intertubular cells wanting. 



This species would be entirely in correspondence with the genus Phaeno- 

 pora of Hall, but the entire absence of intertubular cell-mass, which is 

 always, to some extent, developed in the species of Phacnopora, is a difference 

 of some importance, which, however, will be diminished, after we have seen 

 in Chaetetes species with abundant intertubular cell-mass, and other species 

 composed of tubules alone, with all intermediate forms placed between 

 them. It is also to be noticed, that all the specimens of Chaetetes pavonia 

 which I have seen, appear to be the terminal explanate ends of the fronds, 

 while at the basal ends the cellulose tissue may be devt-loped to some degree. 

 This is decidedly the case in a small ensiform bryozoon of very similar 

 structure, and occurring in the same association. The pointed basal ends of 

 these specimens have a large proportion of cell-mass entering into their 

 structure, while the upper portions are almost exclusively tubulose. 



Chaetetes decipiens, nov. spec. 



Occurs in association with Ch. pavonia, to which it is so surprisingly simi- 

 lar that, even for an experienced eye, it becomes almost impossible to dis- 

 tinguish the two species without the help of a lens. 



It grows in entirely similar thin double leaves ; the surface is covered with 

 the same sort of monticules, composed of larger tubules : the orifices are simi- 

 lar in size and distribution, but a closer examination will reveal sufficient 

 constant differences between the two. 



The latter species has an abundant cell-mass interposed between the tu- 

 bules ; its tube-walls are thin, with not dilated and not contiguous orifices ; 

 the two leaves composing the lamina? are not so clearly defined, and not sep- 

 arable, and on vertical sections the vesiculous cell-rows interposed between 

 the tubules, which themselves are also sometimes septate, will distinguish it 

 at once. 



The thick tube-walls in the one, and the intertubular cell-mass in the other, 

 will produce on the naked eye a similar impression, which disappears under 

 the magnifier. 



This species has likewise much similarity with Ch. frondosus, but it is more 

 delicate in all respects, and in Ch. frondobus the intertubular tissue is consid- 

 erably less developed, its tubules being usually in immediate contiguity. 



The genus Callopora of Hall, comes so near to Chaetetes that it may be well 

 characterized at once, by saying it is a Chaetetes with abundantly developed 

 intertubular cells. Chaetetes Flelcheri, (Milne Edw.) for instance, is in all par- 

 ticulars a Callopora. 



The opercula, described by flail in Callopora eleyantula, are of the same 

 general form as in Chaetetes, but a peculiarity of them is, some five or six 



[May, 



