196 BULLETIN 77, rNITED STATES NATIONAL MUSEUM. 



Genus MESOTRYPA Ulrieh. 



Mesotrypa Ulrich, Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 

 257.— NiCKLES and Bassler, Bull. 173, U. S. Geol. Surv., 1900, p. 30.— 

 Bassler, Bull 292, U. S. Geol. Sui-v., 1906, p. 27.— Hennig, ArcMv. ivx 

 Zool., vol. 4, No. 21, 1908, p. 29. 



Diplotrypa (part) of authors. 



Zoarium hemispheric, conical, or discoidal, generally free, with an 

 epitheca on the under surface; zooecia prismatic or cylindrical, with 

 oblique and sometimes funnel-shaped diaphragms, which are prob- 

 ably modified cystiphragms ; zocecia more or less separated by angu- 

 lar mesopores, which become smaller with age, and are intersected 

 by numerous diaphragms; acanthopores generally present, some- 

 times of large size. 



Genotype. — Diplotrypa injlda Ulrich, Middle Ordovician (Black 

 River) of Minnesota. 



This genus includes several groups of species, all of which agree in 

 having a massive zoarium of zooecial tubes crossed by curved dia- 

 phragms, and of closely tabulated mesopores with rather straight 

 walls. The typical group of the genus includes species with numer- 

 ous curved diaphragms and strong acanthopores, a combination of 

 characters which has not been found well developed in any of the 

 Russian forms, although two new species are referred to this division. 

 A second generic group, of which the American forms Mesotrypa 

 discoidea Ulrich and M.f rotunda Ulrich, from the Mohawkian rocks 

 of Minnesota, are typical, is represented by the very similar Russian 

 form M. discoidea variety orientalis. The latter group, with its few 

 curved diaphragms and absence of acanthopores, suggests alliance 

 with other genera, especially Diplotrypa. A third division of the 

 genus is known only from the specimens here described as M. miUe- 

 poracea, new species, and the new variety parva, in which the absence 

 of acanthopores and the presence of very numerous and exceptionally 

 smaU mesopores are the main peculiarities. 



MESOTRYPA DISCOIDEA ORIENTALIS, new variety. 



Text fig. 106. 



Cf. Mesotrypa discoidea Ulrich, Geol. and Nat. Hist. Surv. Minnesota, vol. 3, 

 pt. 1, 1893, p. 260, fig. 12. 



Zoarium discoid with a flattened, concentrically wrinkled base and 

 a gently convex upper surface; an average example 6 mm. high and 

 20 mm. in diameter. Upper surface smooth, with maculae of large 

 zocecia, inconspicuous, showing plainly only in thin sections. Zooe- 

 cial apertures polygonal, usually hexagonal in outline, with meso- 

 pores occupying the interspaces left between adjoining zooecial walls. 

 Acanthopores apparently absent. Walls of both zooecia and meso- 

 pores thin. Seven to eight zooecia in a distance of 2 mm. 



