6 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 326 



SYSTEMATIC PALEONTOLOGY 



Order TREPOSTOMATA Ulrich, 1882 



Suborder AMALGAMATA Ulrich & Bassler, 1904 



Family MONTICULIPORIDAE Nicholson, 1881 



Genus HOMOTRYPA Ulrich, 1882 



Type species: Homotrypa curvata Ulrich 



HOMOTRYPA EXILIS Ulrich 



Plate 1, figures 1-8 



Homotrypa exilis Ulrich, 1886, 14th Ann. Rept., Geol . Nat. Hist. 

 Survey Minnesota, p. 80; Ulrich, 1895, Geol. Minnesota, 

 v. 3, p. 236, pi. XTX, figs. 10-16. 



External features. — Zoarium ramose, solid, and commonly showing dichoto- 

 mous branching; fragments of zoarium commonly as much as 40 mm long and 12 mm 

 in diameter. Surface smooth, showing maculae composed of mesopores and zooecia 

 above average in size; maculae spaced from 3.0 to 4.5 mm apart, measuring from 

 center to center. 



Tangential section. — Zooecia angular to subangular, commonly with ob- 

 scure outlines; zooecial apertures generally broadly and irregularly ovate, less 

 commonly subcircular or slightly angular; typically (102 of 200 measurements on 

 20 specimens) eight entire zooecia and part of ninth in 2 mm, measuring parallel 

 tolengthot zoarium; commonly (61 of 200 measurements on20 specimens) seven entire 

 zooecia and portion of eighth in this distance; average maximum apertural diameter 

 (based on 200 measurements on 20 specimens and not including zooecial walls) of zoo- 

 ecia 0.21 mm, ranging from 0.15 to 0.26 mm. Zooecial walls thick, usually ranging from 



EXPLANATION OF PLATE 1 



Figures 3 and 5 are X60; all others are X20. 



Specimen number and collecting locality are shown in parentheses follow- 

 ing explanation of each figure. For example, (12P192, 6) refers to specimen 12P192 

 obtained from collecting locality 6. Structures in photographs are not retouched. 



Figure 



1-8 Homotrypa exilis Ulrich . 1 . Longitudinal section; note absence of dia- 

 phragms in outer part of axis and throughout most of submature region 

 and locally developed crenulations in zooecial wall; cystiphragms are 

 more abundant than normal for species (12P192, 6). 2. Small part of 

 longitudinal section from mature zone (12P151, 1). 3. Figure 2 enlarged 

 to show zooecial wall structure in mature zone (12P151, 1). 4. Small 

 area of tangential section (12P199, 6). 5. Figure 4 enlarged to show 

 details of wall structure (12P199, 6). 6. Longitudinal section (12P199, 

 6). 7. Tangential section (12P1 99, 6). 8. Tangential sections of one 

 of nine specimens having more mesopores than customary for species; 

 part of macula shows in top center (12P186, 6) . 



