10 



ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 326 



Text fig. 1 - Measurements on Homotrypa exilis Ulrich. 7 + 

 signifies seven entire zooecia and part 

 of eighth in a 2-mm distance. 



zooecium, probably averaging from eight to 14 per zooecium. True cystiphragms 

 thick, composed of laminae continuous with material of primary wall or secondarily 

 formed cingulum, located in outer part of mature zone, usually not more than three 

 to five per zooecium, and generally found only in a few zooecia in each specimen; 

 cystiphragms arching outward and uniting proximal diaphragm to zooecial wall com- 

 mon in mature region. Mesopores rarely observed, closely tabulate, and confined 

 to mature zone. Acanthopores not seen. 



Remarks. — This species is characterized by its thick zooecial walls, lack 

 of diaphragms in the axis, and the presumable absence of acanthopores; cingula 

 are not uncommon. Mesopores are intercalated sparsely between zooecia in inter- 

 macular areas; in a few specimens, mesopores are distributed sporadically, as 

 they are found sparingly in some areas of a tangential section but are absent in 

 others . 



This is the most abundant ramose form in the Spechts Ferry Shale of north- 

 western Illinois and adjacent parts of Wisconsin and Iowa. The foregoing descrip- 

 tion was based on intensive study of 20 specimens and somewhat more casual ex- 

 amination of 63 other forms. Text figure 1 shows measurements of a number of zooecia 

 in a 2-mm distance and maximum apertural diameter of the zooecia, exclusive of 

 zooecial walls, for the 20 specimens that were studied critically. 



In addition to these 83 specimens, nine others (12P13, 12P23, 12P24,12P33, 

 12P65, 12P73, 12P126, 12P134, and 12P186) are regarded as conspecific although 



