302 BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



Zoarium ramose, growing from an expanded base into more or less 

 branching stems. Zocecial structure as in the Trematoporidse, quite 

 similar to HemipJiragma but differing in having the incomplete dia- 

 phragms practically limited to the bend from the immature to the 

 mature zone. Elsewhere the diaphragms ard complete. Mesopores 

 numerous, moniliform, with tabulae at their constricted portions. 

 Acanthopores developed in two fairly well marked sets, the first 

 consisting of a pair of large acanthopores situated one on each side 

 of a zooecium, and the second of numerous smaller ones distributed 

 indiscriminately along the walls of both zooecia and mesopores. 



The diagnostic generic distinctions of Dittopora, in addition to the 

 general family characters, are believed to be, first the restriction of 

 incomplete diaphragms to the earliest part of the mature zone, and, 

 second, the occurrence of two distinct sets of acanthopores. The 

 internal structure of the genotype, as shown in figure 185, is identical 

 with that of D. annulata, and is equally well developed in Eichwald's 

 Trematopora colliculata, which Dybowski considered a typical species 

 of Trematopora. 



Genotype. — Dittopora clavseformis Dybowski. Middle Ordovician 

 of Esthonia. 



DITTOPORA CLAV-ffiFORMIS Dybowski. 



Plate 3, figs. 7-7 b; plate 11, figs. 18-24; text fig. 185. 



Dittopora clavseformis Dybowski, Die Chaetetiden Ostbaltisclieii Silnr-Forma- 

 tion, 1877, p. 85, pi. 2, figs. 7-76. — Nicholson, The Genus Monticulipora, 

 1881, p. 235, fig. 50. 



The shape of the zoariuco. in this interesting species is so distinct 

 from most other Ordovician bryozoans that the species is easUy iden- 

 tified. Dybowski has given a good description of both the external 

 and internal features, the latter, however, based upon his interpre- 

 tation of the generic characters. As shown in Dybowski's figures on 

 plate 3, the zoarium is of small, club-shaped stems, often with a 

 slightly expanded base. Growth commences upon a foreign object, 

 such as the brachiopod shell (pi. 11, fig. 24), and continues until a 

 colony 10 to 20 mm. high results. With age the basal part of the 

 zoarium becomes covered with elongate, thick-walled mesopores 

 similar to those found in the basal portions of many other bryozoans. 

 Surface of zoarium usually smooth, but occasionally the maculae are 

 so strongly developed that they encircle the branch, and being 

 slightly depressed, give the annulated effect seen in figure 22 of 

 plate 11. Zoarial growth usually stops with the formation of a single 

 club-shaped mass, but occasionally a second similarly shaped zoarium 

 arises from the upper portion of the first. Rarely the initial zoarium 

 bifurcates (see pi. 11, fig. 23). 



Zooecia small, elongate-oval in young specimens but angular, 

 thick-walled in the more mature condition. Mesopores numerous, 



