6 72 PALAEONTOLOGY OF ILLINOIS. 



surface with an epithecal membrane, marked with irregular con- 

 centric wave-like wrinkles and faint radiating- lines, the latter 

 being the base of the zocecia walls showing through; at the 

 center there is a slightly raised or depressed cicatrix of attach- 

 ment, about 5 mm. in diameter. Upper surface occasionally 

 even, generally with more or less elevated monticules, composed 

 of cells wider apart and rather larger than the average. Zoce- 

 cia for a short distance prostrate and thin-walled, then making 

 a rather sharp curve, proceed direct to the surface; the walls 

 have now become thicker and a number of mesopores devel- 

 oped, which may be intersected, according to age, by from one 

 to four or more centrally perforated thick diaphragms. The 

 zooecial walls exhibit a large number of delicate transverse 

 lines, which may represent minute connecting foramina. Some 

 of the zooecia also show several tooth-like processes projecting 

 a short distance into the vestibular cavity. These denticles 

 seem to have been developed at successive levels, perhaps in all 

 the zooecia. Apertures sub-circular, the posterior side usually 

 somewhat straightened, averaging 0.15 mm. in diameter, sur- 

 rounded by a thin and but little elevated peristome, that, some- 

 times at any rate, carries a number of very small granules. 

 Between the monticules they are arranged in quincunx, thus 

 forming transverse, longitudinal and diagonally intersecting 

 rows, whose regularity is more or less disturbed around the 

 monticules. The triangular or oblong quadrate space left be- 

 tween the apertures is occupied by one or two mesopores, either 

 open or closed by a membrane. 



Three specimens give the following measurements: 

 No. 1. Transversely, 11 apertures in 3 mm. 



Eleven of the transverse rows in 3 mm. 



Diagonally, 10 apertures in 3 mm. 



Total number of apertures in 3 mm. square, 121. 

 In this specimen the interspaces between the apertures are 

 triangular as in fig. 3d. It is from the Keokuk limestone, at 

 Keokuk, Iowa. 

 No. 2. Transversely, 9 apertures in 3 mm. 



Fourteen of the transverse rows in 3 mm. 



Diagonally, 12 apertures in 3 mm. 



Total in 3 mm. square, 126. 



