EAKLY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 301 



importance in so simple a form, but the Middle Ordovician form is 

 undoubtedly the progenitor of the early Silurian genotype, and the 

 purposes of classification may, perhaps, be best furthered by con- 

 sidering it as only a variety. 



Occurrence. — Found in the Wassalem beds (D3) at Uxnorm and 

 Gut Sack, and in the Wesenberg limestone (E) at Wesenberg, 

 Esthonia. 



Eolotype.— Cat. No. 57387, U.S.N.M. 



Fragment and thin sections of the type-specimen in the collections 

 of the British Museum. 



Genus DITTOPORA Dybowski. 



Dittopora Dybowski, Die Chaetetiden der Ostbalt. Silur-Formation, 1877, p. 84. — 

 Nicholson, Genus Monticulipora, 1881, p. 234. — Roemer, Lethsea geog- 

 nostica, pt. 1, Leth. Pal., 1883, p. 479. 



Trematopora Dybowski, Die Chaetetiden der Ostbalt. Silur-Formation, 1877, 

 p. 69. 



This generic name was proposed for certain monticuhporoids in 

 which, according to Dybowski, the corallites were of two kinds, one 

 being cylindrical and separated by a reticulate coenenchyma, and 

 the other consisting of prismatic tubes in close contact. These two 

 kinds of corallites, according to the same author, may be arranged 

 in alternating rows or may be confined to particular parts of the 

 colony. Wandrohrchen (acanthopores) and tabulae are present. 

 In the genotype, I), clavxformis Dybowski, in which the colony is 

 club-shaped, the prismatic coralhtes are restricted to the basal por- 

 tion, while the cyhndrical kind of corallites occur in the upper 

 portion of the stem. In a second species, D. annulata (Eichwald), 

 the two kinds of corallites are arranged in alternating, transversely 

 elongated zones. 



Both of these species are well represented in the collections before 

 me, and from a study of them, I must conclude that Dybowski's 

 generic characters are valueless — indeed the same features are found 

 in many diverse genera. The cyhndrical corallites separated by a 

 reticulate coenenchyma are simply the ordinary zooscia with inter- 

 vening, closely tabulated mesopores. The prismatic tubes in close 

 contact are thick-walled mesopores which, in D. clavseformis are 

 extremely abundant at the base of the zoarium, and in D. annulata 

 form the ring-hke maculae. Thin sections of the two species men- 

 tioned, and of the very abundant Trematopora colliculata Eichwald, 

 indicate that these European forms constitute a natural group closely 

 related on one hand to Trematopora and on the other to Hemi- 

 phragma. It is, therefore, deemed advisable to retain the name 

 Dittopora for this group of species, with the following amended 

 definition. 



