310 BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



In Monotrypa the simplest structure of the Trematoporidse is 

 present. The zoarium is massive and composed of simple, thin- 

 walled, polygonal zooecia in contact, both mesopores and acantho- 

 pores being absent. Internally the wall structure is as in other 

 trematoporoid genera, even to the crinkling so often noted in the 

 family. The only species so far found in the Russian collections 

 seems to be closely allied to the genotype. 



Genotype. — Chsetetes undulatus Nicholson. Middle Ordovician of 

 Canada. 



MONOTRYPA JEWENSIS, new species. 



Text fig. 191. 



Cfr. Monotrypa undulata Nicholson, Paleozoic Tabulate Corals, 1879, p. 321, pi. 

 14, figs. 3-36, 4, 4a. 



Zoarium of rounded, hemispherical, or depressed spherical masses 

 10 to 15 mm. in diameter and also in height. Celluliferous surface 

 smooth, with thin-walled, polygonal zooecial apertures, four to five in 

 2 mm. Clusters of zooecia larger than usual present at regular inter- 

 vals but inconspicuous. Occasionally small, angular mesopore-like 

 openings are scattered among the ordinary apertures, but these rep- 

 resent young zooecia. 



The important features of vertical sections are the crenulations of 

 the zooecial walls and the practical absence of diaphragms both in. 

 the immature and mature zones. Occasionally a single diaphragm 

 is inserted ia the outer portion of the mature zone as represented in 

 figure 191 c. Tangential sections show the extreme simplicity of 

 structure of the thin-walled, angular zooecia. 



The massive habit of growth, thin-walled polygonal zooecia, crenu- 

 lated walls, and practical absence of diaphragms are characters which 

 will distinguish this species from all associated forms and cause it to 

 be recognized without difficulty. A vertical fracture is of course 

 necessary to determine the more important characters. 



Upon comparison with American forms, Monotrypa jewensis is 

 found to be closely related to the type of the genus, M. undulata, 

 from the lower Trenton strata of Canada. The latter is not as well 

 known, as it should be, but, judging from Nicholson's description 

 and figures, M. undulata has slightly smaller zooecia and more abun- 

 dant diaphragms. 



Occurrence. — Not uncommon in the Jewe limestone (Dl), Baron 

 Toll's estate, and in the Kegel limestone (D2), at Kegel, Esthonia. 



Eolotype.—C&t. No. 57410, U.S.N.M. 



British Museum, specimen and thin sections from the Jewe lime- 

 stone, Baron Toll's estate. 



