EAULY PALEOZOIC BEYOZOA OF THE BALTIC PEOVINCES. 185 



Numerous typical species of this interesting genus occur in the 

 faunas of late Trenton and of Cincinnatian ages, believed to hare been 

 derived from the south Atlantic. The northern Atlantic or Arctic 

 types occur chiefly in the Black River and earhest Trenton forma- 

 tions, and in America are known mainly from deposits in Minnesota 

 and the neighboring States. Of these latter species, Homotrypa sub- 

 ramosa Ulrich and H. similis Foord are the most abundant and wide- 

 spread species in America, and their geographic range has now been 

 extended to the Baltic region by the discovery of specimens in several 

 of the Russian formations. Each of these two species is quite unlike 

 the Cincinnatian type of the genus and may be taken as typical of a 

 subgeneric division. Thus, Homotrypa similis shows a tendency to 

 variation toward Eridotrypa, while the acanthopores and the cysti- 

 phragms of H. subramosa recall Monticulipora. 



The zoarium in Homotrypa is of ramose or frondescent, smooth or 

 monticulated branches made up of polygonal zooecia with thin, 

 finely crenulated walls and few diaphragms in the axial region, and 

 thicker walls lined by cystiphragms in the peripheral zone. Meso- 

 pores are either absent or are restricted to the maculae, but acantho- 

 pores are generally present. The ramose forms of Monticulipora 

 have great similarity to Homotrypa externally, but a thin section of 

 the former shows a granulose wall structure very different from the 

 clearer, more distinct walls of Homotrypa. 



Genotype. — Homotrypa curvata Ulrich. Upper Ordovician (j\'Iays- 

 ville), Ohio Valley. 



HOMOTRYPA SIMILIS Foord. 



Text figs. 97, 98. 



Homotrypa similis Foord, Contr. Micro-Pal. Cambro-Sil., 1883, p. 10, pi. 2, 

 figs. 2-2d. — Ulrich, Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 

 1893, p. 242, pi. 20, figs. 28-33. 



This characteristic American Lower Trenton bryozoan is repre- 

 sented in the Russian Ordovician by numerous specimens from the 

 Wassalem beds (D3) at Uxnorm. Careful comparison show these 

 examples to agree with the American types in all respects save that 

 as a rule the zoaria of the Russian specimens are better developed 

 and have less obhque zooecia. This latter fact is of slight importance, 

 since the study of the Paleozoic Bryozoa has shown that the greater 

 the development of the mature zone the more directly do the zooecia 

 open upon the surface. The essential characters of Homotrypa 

 similis are as follows : 



Zoarium of subcylindrical or compressed branches 4 to 10 mm. 

 in diameter, dividing unequally and often so irregularly as to anasto- 

 mose. Surface smooth, with distinct, substellate maculae composed 



