/ BRYOZOA. 255 



Aspldopora.] 



Taking Aspidopora in the sense above described, there is only one character 

 (the other peculiarities being dependent upon it) that will distinguish it from 

 Prasopora, namely, the thinness of the zoarial expansion and the consequent brevity 

 of the zooecial tubes. That this is the mature condition of the zoaria is proved by 

 finding hundreds of examples of some of the species, not one of which exceeds 1 mm. 

 in thickness, though many of them may be over 25 mm. in diameter. Some may 

 consist (see plate XVII, fig 17) of several layers in contact at limited points but 

 really quite distinct from each other, and thus showing that, beyond the develop- 

 ment of the individual layers, the method of growth is not the same as in Prasopora. 

 In that genus the tubes, though perhaps presenting many immature and mature re- 

 gions or layers, are continuous thrbught the zoarium, even if it be over 50 mm. in 

 thickness. The- tabulation of the two sets of tubes is essentially the same in the 

 two genera, excepting that in Aspidopora it is altogether as in the immature regions 

 of a Prasopora* Perhaps some significance is to be attached also to the fact that 

 only a few cystiphragms and no straight diaphragms occur in the zooecial tubes. 



Seven species are referred to Aspidopora as now defined. The next described,- 

 A. parasitica, is the oldest, and occurs in the lower and middle thirds of the Trenton 

 shales. This species is also one of the earliest known phases of the type of structure 

 that at this time was evidently being rapidly differentiated into true Prasopora and 

 Mesotryva. Before we can fully understand the relations of these groups of species 

 to each other it is necessary to discover the more primitive forms that are, to 

 be expected in the Chazy. The second species, A. elegantula, occurs in the Gralena 

 shales, and the five remaining, A. areolata Ulrich, A. newberryi Nicholson, sp., A. caly- 

 cula (James) Nich., sp., A. eccentrica James, sp., and an undescribed species, in the 

 lower two hundred feet of strata at Cincinnati, Ohio. 



Aspidopora pakasitioa Ulrich. 



PLATE XVII, FIGS. 26-32. 



Aspidopora parasitica (part.) Ulkich, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 90. 



Zoarium parasitically attached to shells, Streptelasma profundum Conrad, and 

 other foreign bodies, upon which it forms very thin, subcircular or irregular 

 patches, 10 to 20 mm. in diameter, and 0.5 mm. or less in thickness. -Zooecial walls 

 very thin, apertures oval or circular, arranged in regular curved series around 

 groups of cells distinctly larger than the average ; eleven or twelve of the latter in 



*I nrevious putiUcations on this genus (loc. ctt.) I did not mention the presence of cystiphragms iDecause these structures 



rl t be wanting in the type species. My present opinion Is that J,he supposed diaphragms figured by me for A. mrfolata, 

 (op"it , vol. vl, p. 164), are really cystiphragms, in part Incorrectly drawn. 



