﻿American Palceozoic Fossils. 



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plate such sections, prepared from authentic examples of R. gracile, 

 are represented by figures 8 and 8a. Compared with Rhomb op or a, we 

 find that Rhabdomeson differs from species of that genus in only a 

 single character to which generic importance is attributable. This is 

 found in the central tubular axis, to which the proximal ends of the 

 zooecia are attached, and from which they radiate in all directions. 

 In Rhombopora no such axis exists, and the zooecia radiate in all 

 directions from an imaginary axis, the same as they do in all of the 

 ramose Monticuliporidce. While sections of the two genera are there- 

 fore readily distinguished, it is, on the other hand, not nearly so easy 

 to separate them by their superficial characters. Indeed, examples of 

 Rhombopora persimilis, n. sp. from the Kaskaskia group of Ken- 

 tucky and Illinois, resemble specimens of Rhabdomeson gracile so 

 closely in growth, zocecial apertures, and distribution of spines, that 

 it requires no little amount of patience to distinguish them success- 

 fully. 



The family Rhab domes ontidce is an unquestionable member of the 

 Trepostomata, and finds its nearest allies in the section of the Mon- 

 ticuliporidce typified by Batostomella, Ulrich. That genus, as well as 

 all other members of that family, differs considerably in the structure 

 of the peripheral or "mature" portion of the zooecia. While the evi- 

 dence on the point is not positive, it is still not unreasonable to 

 believe, that in at least some of the Rhab domes ontidai the zocecial 

 cavities communicated with the spiniform tubuli, and thus with each 

 other, by means of a large number of very minute canals. A central 

 tube, such as marks Rhabdomeson, occurs in Anisotrypa, and at least 

 in two other Palaeozoic genera of Bryozoa, of widely different affinities, 

 the one (Cceloclema, Ulrich) belonging to the Ceramporidce, and the 

 other (Cheilotrypa, n. gen.) to the Fistuliporidai. In Rhabdomeson 

 it is straight and of nearly equal diameter throughout, while in the 

 other genera it is irregularly constricted and expanded. This differ- 

 ence is doubtless mostly due to the facts that in Rhabdomeson the 

 zoarium is much more slender, and the cell development, and conse- 

 quently the general growth of the branches, is more regular than it is 

 in either Anisotrypa, Codoclema or Cheilotrypa. 



Mr. F. B. Meek, the author of Rhombopora, regarded the genus as 

 belonging to the Polypi. Against this view I will simply sa t y, that if 

 Rhombopora is a coral, then we have no Bryozoa in American Palaeo- 

 zoic rocks. 



