﻿American Palaeozoic Fossils. 



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now known are generally due to its influence. Practice, however, 

 proves that such a classification can not even be approximately 

 natural when applied to Palaeozoic B^ozoa. It seems that in those 

 early times their characters had not yet attained the stability that 

 marks the more recent forms, and makes their classification, compar- 

 atively, an easy task. Among the Palaeozoic Bryozoa almost any 

 characters may, for a time, be endowed with great importance, and 

 its value can only be determined by ascertaining its degree of per- 

 sistency, and comparing it with that of the other characters. Thus 

 by obtaining the average of importance, if the expression be allowed, 

 a very nearly natural classification is the result. In the proposed 

 fam. Cystodictyonidce we have a combination of characters not occur- 

 ring in any other group, while, on the other hand, not one of these 

 can be said to pertain to this group alone, nor to be especiallv dis- 

 tinctive. The systematic position of the family appears to be between 

 the Stictoporidce and the Fistuliporidai. From the former it differs 

 in having the interstitial spaces occupied by vesicular tissue, the cell 

 apertures provided on one side with a lip, and the consequent two 

 vertical ridges which project into the visceral cavity of the tubes. 

 From the Fistuliporidai the new family differs mainly in possessing 

 two or more leaved zoaria, the margins of which are non -poriferous, 

 and usually sharp. 



Cystodictya, Ulrich, 1882. (Ante vol. v., p. 152 et 170.) 



Arccmopora, Vine. Fourth Rep. to Brit. Ass. on Fossil Polyzoa, 1883. 



Zoaria composed of two layers of cells grown together back to back, 

 assuming the form of dichotomously divided, compressed branches, 

 with an acutely elliptical transverse section, and sub-parallel, sharp, 

 non -poriferous margins. Zooecia tubulax, prostrate at first, then 

 abruptly assuming an erect position. Superficially they are arranged 

 in regular longitudinal and diagonally intersecting series, with sub- 

 circular or sub-pyriform apertures, on the outer side of which the 

 margin is elevated to form a small lip. The ends of this lip project 

 slightly into the cell cavity, gradually forming, as growth proceeds, 

 two slender ridges, the lower one being developed a little earlier than 

 the upper (see Pi. II., figs. 3, 4c, and 5). Zooecial apertures closed by 

 centrally perforated opercula. Inter-apertural spaces smooth or finely 

 striated. Tangential sections, passing through the "immature" 

 region of the zoarium, show that the zooecia are ranged in longitu- 

 dinal series between vertical plates, to one of which they are later- 

 ally attached, while the intervening spaces are occupied by irregularly 



