BRYOZOA. 155 
Pachydictya.] 
The internal characters, which in most respects remind us greatly of P. occident- 
alis, surely more of that species than of P. acuta, were obtained from thin sections 
of one of the last mentioned specimens. 
Compared with other species P. occidentalis offers many points of agreement, 
but, so far as known, is distinguished readily enough by its macule, and the less 
regular arrangement of its zocecia and inter-apertural markings. P. fimbriata is 
also closely related, but the peculiar wavy character of its borders serves well in 
separating them. In P. acuta and varieties the spaces separating the rows of 
apertures are more ridge-like, and the end spaces longer. The branches also are, 
except in rare instances, narrower.* 
Formation and locality—Not uncommon in the Galena shales at St. Paul, Minnesota, where it is 
associated with an abundance of Zygospira recurvirostris (Hall) and segments of Arthroclema. Arthropora 
reversa is found on the same slabs of rock. Also at Decorah, Iowa. 
Mus. Reg. No. 7596. 
Pacuyprotya acuta Hall, and varieties. 
PLATE VIII, FIGS. 11-17; PLATE IX, FIG. 7. 
Stictopora (?) acuta HALL, 1847. Pal. N. Y., vol. i, p. 74, pl. xxvi, figs. 3a, b. 
Stictopora or Ptilodictya acuta (part.) of many authors. 
Stictopora acuta ULRICH, 1882. Jour. Cin. Soc. Nat. Hist., vol. v, p. 168, pl. viii, figs. 1, la, 1b. 
Pachydictya acuta ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat Hist. Sur. Minn., pp. 75 and 76. 
(Merely mentioned as a species of Pachydictya.) 
This so frequently yet so often incorrectly quoted species, has given me no little 
trouble, first, because of the difficulty of determining exactly what species Hall 
originally intended, and second, because of its variability. The species might be 
subdivided, but I doubt the advisability of doing so, since most of the varieties are 
exceedingly difficult to recognize. The species, with all its varieties, is also restricted 
to the Trenton limestone, or rocks equivalent to that horizon. Hence, we have not 
the usual though good excuse for proposing varietal distinctions. The species is to 
_be regarded as one of the most characteristic and widely distributed fossils of the 
Trenton proper, being also abundant at many localities in New York, Vermont, 
Canada, Kentucky, Tennessee, Illinois, Wisconsin, Minnesota, and Manitoba. It has 
been reported to occur in the Birdseye and Black River horizons, but that is most 
likely an error, species of Rhinidictya, which abound in those rocks, having but too 
often been confounded with P. acuta. 
Figure 11 of Plate VIII, represents one of seven fragments from the original 
locality, Trenton Falls, New York, which I owe to the kindness of \ r.C. D. Walcott. 
Its surface magnified nine diameters is shown in fig. 12 of the same plate. In this 
= 2a thouen now obliged to regard P, elegans as specifically distinct, I expect, with material soon to be gathered, to be able 
to show that it is merely a later development of P. occidentalis, Perhaps also that it is really an intermediate stage between 
that species and P. acuta, 
