BRYOZOA. 581 



Bryozoa. The relation to Polypora is in every way equivalent 

 to that between Archimedes and Fenestella, the only difference 

 being that while in Archimedes the stony support forms spiral 

 volutions, in Lyropora it is Y- or U-shaped. The form and 

 other peculiarities of the support are quite const ant J*f or each 

 species, and when this portion of the zoarium happens to be 

 similar in two species, as for instance in L. quincuncialis Hall, 

 and L. ranosculum Ulr., and L. subquadrans Hall, and L. 

 divergens Ulr., marked differences are found in the struc- 

 ture of the net -work. This is more or less different in all the 

 cpecies. and I have found it possible to determine with certainty 

 the specific relations of the merest fragments. 



Six species, and possibly two more, beside one variety, are 

 known to possess the characteristic support of the genus. With 

 the exception of L. i*etroi-sa M. & "W., a rather rare species in 

 the Burlington limestone, all are from the Chester group. 



LYKOPORA RANOSCULUM Ulrich. 



PL LVm, Fig. l-lc. 



Zoarium large, free, a foliate expansion spread between the 

 recurved ends of a strong support. Support parabolic, the 

 arms diverging at a variable degree, subcircular or oval in 

 cross section, consisting of exceedingly thin, transversely grano- 

 striate concentric layers of sclerenchyma. which are successively 

 deposited upon the lateral margins of the strongly convex 

 fenestrated expansion At the base of the support there is com- 

 monly an obtuse knob-like thickening. Fenestrated expansion 

 celluliferous oh the convex side. Here the branches vary in 

 thicknegs from 0.4 to 0.6 mm., with about 15 in one cm.; their 

 surface rounded and, when perfect, with a somewhat irregular 

 series of strong tubercles, two or three to each fenestrule, along 

 the middle; the zooecia with circular or sometimes irregularly 

 inflected apertures, about 0.09 mm. in diameter, separated by 

 interspaces as wide or wider, and arranged in from three to five 

 (usually four) only moderately regular alternating ranges, 21 

 or 22 in five mm.; the fenestrules irregularly oval, eleven in one 

 cm.; the dissepiments short, their width as great or greater 

 than the length of the fenestrules. On the reverse the fenes- 

 trules are circular or round oval and larger than on the ob- 



