BRYOZOA. 163 



I'tilodlctya.] 



moved, for in that case we would have one easily recognized though not peculiar 

 character running through the family that is not represented in Stidoporella and 

 allied genera.* A basal articulation, namely, pertains to Ptilodidya, Escharopora, 

 Phcenopora, Clathropora, Graptodidya, and Arthropom, while in Stidoporella and 

 genera of that type, the zoarium is continuous throughout, and attached below in 

 the ordinary manner, i. e. by a simple basal expansion forming one piece with the 

 erect frond. 



If removed from the Ptilodidyonidce it would be necessary to establish a new family 

 for their reception, since they cannot, because of the absence of median tubuli 

 between their mesial laminae, be placed with the Rhinidictyonidce, the only remain- 

 ing family of paleozoic Bryozoa with which they have any affinity. It was because 

 they agree in this and most other respects with Escharopora, that I arranged them 

 with the more typical Ptilodidyonidie. The new family would hold an intermediate 

 position between the Rhinididyonidw and Ptilodidyonidie, differing from the former 

 in its zooecial characters, and from the latter in its continuous zoarium, presumably 

 a zoarial modification.! 



Genus PTILODICTYA, Lonsdale. 



Flustra (part.), GOLDFUSS, 1826. Petref. Germ. 



Ptilodictya, LONSDALE, 1839, Murch. Sil. Syst., p. 676. 



Ptilodictya (part.), NICHOLSON, 1874. Geol. Mag., n. s., vol. i, p. 123, and Pal. Oat., p. 97; VINE, 1881 



Second Brit. Assoc. Rep. Foss. Poly. , Quar. Jour. Geol. Soc. Feb. 1882, 

 and 1884, Fourth Brit. Assoc. Rep. Foss. Pol., p. 37; CLRICH, 1882, 

 Jour. Gin. Soc, Nat. Hist., vol. v, p. 151, and 1890, Geol. Surv. 111., 

 vol. viii, p. 390; HALL, 1887, Pal. N. Y., vol. vi, p. 19. 



Escharopora, HALL, 1874 and 1879. Twenty-sixth and Thirty-second Rep. N. Y. State Mus. Nat. 



Hist. (Not 1847, Pal. N. Y., vol. i.) 



Heterdictya, NICHOLSON, 1875. Geol. Mag., and Pal. Ont., (ii) p. 79. 



In my preliminary report on the Bryozoa of Minnesota (Fourteenth Ann. Rep. 

 Geol. Nat. Hist. Surv. Minn., p, 64 ; 1886) I mention the two sections into which 

 Ptilodidya, as understood by me in 1882 (loc. cit.), may be divided. Since then I have 

 given the subject further study, with the result that I now believe they may be dis- 

 tinguished in a generic sense. 



*An articulated zoarium is of rather cpmmon occurrence among both the living and extinct Bryozoa. Of Paleozoic 

 types the Arthro*tyHdas and true PtilodictyonidcE are the best representatives of this method of growth. It Is also character- 

 istic of Acroacnia, Hall, and Dicraiiopora, ITlrieh. 



+ In drawing this distinction the systematist is once more called upon to decide between zoarial and zowcial varia- 

 tions as furnishing the best and most reliable tests of relationship. The more I study these questions of relationship, the 

 less practical seems the adoption of strict rules for our guidance in the delimitation of the classiflcatory sections whereby 

 wo attempt to express our ideas of natural modifications. What may appear as, and probably is. sulticU'nt ground for the 

 erection of a genus or family in one case, does not necessarily suffice in another. There are so many points to be taken into 

 account before anything even approximately expressing nature's handiwork can result. Among them, environment, asso- 

 ciation, and relative position in the geological scale, are of great importance. The last, If judiciously used, is always an 

 excellent clue to relationship, and one that has been but too rarely taken into consideration by students of recent zoology. 

 Volumes are to be written upon these intricate questions, but I have said enough probably to show that a successful classi- 

 fication cannot be worked out in a day, nor is any yet drawn up that will not suffer greater or less modification in time. The 

 stability of a classification depends not a little upon the collector, since it is his discoveries that build it up or tear it down. 



