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 maculae, and the less 

 regular arrangement of its zooacia 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 aiid 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. 



PACHYDICTYA ACUTA Hall, and varieties. 



PLATE VIII, FIGS. 11-17; PLATE IX, FIG. 7. 



Stictopora (?) acuta HALL, 1847. Pal. N. Y., vol. i, p. 74, pi. xxvi, figs. 3a, b. 

 Stictopora or Ptilodictya acuta (part.) of many authors. 



Stictopora acuta ULKICH, 1882. Jour. Gin. Soc. Nat. Hist., vol. v, p. 168, pi. viii, figs. 1', la, Ib. 

 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 propei', 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 Bhinidictya, which abound in those rocks, having but too 

 often been confounded with P. acuta. 



Figure 1 1 of Plate VIII, represents one of seven fragments from the original 

 locality, Trenton Falls, New York, which I owe to the kindness of A.r.C. D. Walcott. 

 Its surface magnified nine diameters is shown in fig. 12 of the same plate. In this 



* Though now obliged to regard P, elegants 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. 



