232 Ulrich on Genera of 'Bryozoa. 
ada. Besides the type species eight others are known to me 
which are constructed upon the same general plan, four of them 
Lower Silurian and four Middle and Upper Silurian. These 
are distributed as follows: H. spiniformis Ul., Birdseye, II. 
divaricata UL, Trenton, H. imbricata Ul. and H. ^arr/^/ James, 
Cincinnati group, H. bellula Bill., H. annata^ Bill, and H. no- 
dosa^ Bill.,' Anticosti group, and H. lindstrovii n. sp. from the 
Upper Silurian rocks of Gotland. 
Upon comparison I find that the Lower Silurian species differ 
from the Upper Silurian and typical section of the genus in 
having the cell-apertures arranged in longitudinal series between 
elevated ridges, and the interspaces between the ends of the 
zooecia apertures longer. In H. frag His the apertures are rather 
oblique, oval or subquadrate, and separated by comparatively 
thin subequal walls. In the Gotland species they are ovate, 
nearly or quite direct, and surrounded by a hexagonal margin. 
The Upper Silurian and Anticosti species also have small acan- 
thopores, which, so far, have not been detected on any of the 
Lower Silurian forms. In short the latter compare more closely 
in the arrangement of the zooecia with Arthroclema Billings 
than do the typical species. 
It gives me great pleasure to name this species as above, in 
honor of the talented Swedish palaeontologist. Dr. Gustav Lind- 
strom, to whose kindness I owe the opportunity of studying 
this beautiful species. The specimens are more perfect than 
those of any other species of Helopora yet seen by me. 
In Arthroclema^ Billings, the arrangement of the zooecia is, 
as has already been intimated, very much the same as in the 
Lower Silurian species Helopora^ but the development and com- 
bination of the segments in the three species known to me is so 
peculiar that the distinctness of the two genera can scarcely be 
questioned. In Arthrocletna^ namely, the zoarium is composed 
■of a large number of subcylindrical segments. These are ar- 
1 In the Catalogue Sil. JFoss. Anticosti^ Mr. E. Billings describes twelve 
species which he refers to Helopora. I have not seen specimens of all of 
these but from a study of the descriptions I believe I can say safely that 
with the exception of the three species above mentioned, and possibly H, 
formosa and H. coticava, none of the others are congeneric with H.fragilis. 
H. lineata and perhaps H. strigosa and H. striatopora belong to Nemato- 
fora; the others are undetermined. 
