566 PROCEEDTNGS OF THE NATIONAL MUSEUM. vol. xxvi. 



nicel}^ brought out by smoothing the surface of the zoarium with the 

 edge of a knife bhide or rubbing upon a gritty stone, and after etching 

 slightly with acid examining with a lens the spot thus treated, mois- 

 tening it slightly. Similarly, vertical fractures when treated in the 

 same way show the characters seen in vertical sections. By this ready 

 method it is seldom difficult to recognize a Tlomotrypa as the cysti- 

 phragms in the peripheral region are easily detected, if not in the 

 tangential, then certainly in the vertical section. Except in the mat- 

 ter of size, the surface characters of the zonecia are seldom distinctive 

 of any species of Jlomotrypa and are more or less similar in all the 

 species. For that reason, only the zoarium and internal characters of 

 the species here described are figured and only when the surface char- 

 acters are out of the ordinary are they described. To obtain the 

 number of zooecia in a given space a measurement is made from the 

 center of one macula to the center of one adjoining. The average of 

 several such measurements gives the correct number, which may be 

 verified by counting the number of zooecial tubes in the same space 

 in the peripheral region of vertical sections. 



HOMOTRTPA Ulrich. 



Homotrypa Ulrich, Jour. Cincinnati Soc. Nat. Hist., V, 1882, p. 240; Geol. Sur. 

 Illinois, YIII, 1890, pp. 370, 409; Geol. Minnesota, III, 1893, p. 235; Zittel's 

 Textb. Pal. (Engl, ed.), 1896, p. 273.— Fooed, Contr. Micro.-Pal. Cambro.- 

 Sil., 1883, p. 9.— Miller, North American Geol. Pal., 1889, p. 309.— Nickles 

 and Bassler, Bull. U. S. Geol. Survey, No. 173, 1900, p. 29. 



The genus has been briefly defined as follows: 



Zoarium frondescent or ramose; maculse or monticules of larger cell apertures a 

 characteristic feature; apertures often oblique; zocecia with very thin or finely crenu- 

 lated walls and remote diaphragms in immature region and cystiphragms, isolated or 

 in series, confined to mature region; mesopores few, in clusters; acanthopores gen- 

 erally developed." 



The essential generic characters are the upright zoarium, the pres- 

 ence of cystiphragms in the peripheral region only, and the develop- 

 ment of few mesopores. The form of the zoarium, the shape and size 

 of the maculte and zooecia, and the number of the latter in a given 

 space, thickness of zooecial Avails, distribution of diaphragms and 

 cystiphragms, and the number, size, and distribution of acanthopores 

 and mesopores are the important variable quantities upon which ther 

 specific characters are based. 



The species of Jlomotrypa may be classed into two well-defined 

 groups, the presence or absence of diaphragms in the peripheral 

 region of the zooicial tubes being the distinguishing chai-acteristic. 

 In the typical section, which may be designated the JI. curvata group. 



"Nickles and Bassler, Bull. U. S. Geol. Survey, No. 173, 1900, p. 29. 



