198 BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



tabulation of the zooecia, which, in the Minnesota species, is closer and 

 often exhibits curved diapliragms. A second but minor difference 

 may be noted in the shape of the zooecia, which in M. discoidea are 

 often rounded and in the variety orientalis are most often hexagonal. 

 The latter is apparently the European representative of the American 

 species, and the reference of both to Mesotrypa, in spite of their 

 numerous straight diaphragms, is believed to be in keeping with the 

 development of the genus. As remarked under the generic discus- 

 sion, these two forms and the following new species, in addition to 

 Mesotrypa rotunda Ulrich, form a small group differing from typical 

 forms of the genus in having straight diapliragms more often than 

 curved. In other respects, namely, the distinctly straight, well- 

 defined walls of both zooecia and mesopores and the close, compact 

 tabulation of the latter, the generic characters are those of Mesotrypa. 



Of associated species, Orhipora distincta has a similar zoarium but 

 its zooecia are much larger, acanthopores are numerous, and meso- 

 pores are wanting. Diplotrypa hicornis often has a similar external 

 aspect, but the difference in tabulation may be seen by comparison 

 of the text figures here given. This latter species likewise has the 

 moniliform mesopores and loose tabulation of Diplotrypa. 



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

 Toll's estate, near Jewe, at St. Mathias, and Paeskiill, Esthonia. 



Coty pes. —Ca.t. No. 57280, U.S.N.M. 



Specimens and a thin section of the type-specimen are in the col- 

 lections of the British Museum. 



MESOTRYPA EGENA, new species. 

 Text fig. 107. 



The species for which the above new name is proposed is so much 

 like the preceding externally that an examination of its internal 

 structure is necessary for its identification. Both species agree in 

 having a discoid zoarium with a flat, concentrically wrinkled base 

 and a gently convex upper surface. Specimens of Mesotrypa dis- 

 coidea orientalis as a rule exhibit numerous mesopores at the surface, 

 while in M. egena mesopores are so few that the zooecia are polygonal 

 and in contact. The surest method of distinguishing the two, how- 

 ever, is by an examination of the tabulation, as shown in vertical 

 fracture or thin section. In the present species the zooecial dia- 

 phragms are from one to two times their own diameter apart in the 

 immature zone, but so crowded in the mature region that four dia- 

 phragms sometimes occur in a distance equal to their own diameter. 

 In these crowded zones the diaphragms are frequently so curved as 

 to resemble cystiphragms. In M, discoidea orientalis the diaphragms 



