346 PALEONTOLOGY OF ILLINOIS. 



I have termed the superior hemiseptum. The inferior hemisep- 

 tum is less frequently met with. When present it springs from 

 the bottom of the cell or from near the base of the anterior 

 wall. The disposition of parts will be better understood by 

 referring to the diagrammatic section of the cryptostomatous cell 

 given in fig. 9(p. 330); see also fig. 10 (p. 351), and, on plate LV, 

 figures 7 and 8: The vestibule is the cylindrical portion of the cav- 

 ity that is included between the external aperture and the super- 

 ior hemiseptum. The latter is best developed in forms having a 

 sloping area around the external orifice, yet, even among these, 

 (e. g. Stictoporella, fig. 13,) it may be quite obsolete or entirely 

 unrecognizable. When the external aperture is surrounded by a 

 peristome the superior hemiseptum is nearly always wanting, 

 but the inferior may be present. 



The material which I propose to arrange under this suborder 

 is divided into eleven families, comprising together seventy or 

 more genera, some of them very prolific. The PTILODICTYONID^E 

 may be regarded as the most typical. 



(1) CYSTODICTYONHLE: In this family the zoaria are composed 

 of two or three layers of cells, that grew together back to back, 

 forming by their union either bifoliate expansions or triangular 

 branches, the particular zoarial habit assumed by the various 

 genera being very constant. The margins are non-poriferous 

 and finely granulo-striate, while the zocecial apertures over the 

 basal portions become gradually covered by the same kind of 

 calcareous deposit. The form of the primitive cell is probably 

 the most distinctive feature of the family, being semi-cordate or 

 obovate-acuminate in outline, both forms being found in the 

 same example. They are always arranged in longitudinal series, 

 between vertical plates, usually alternating and, either entirely 

 separated from each other, or in contact at only limited por- 

 tions of their circumference, the intermediate spaces being 

 occupied by vesicles. The primitive orifice was terminal, of 

 sub-circular form and somewhat eccentric, while the front of the 

 cell Avas slightly convex, and appears to have been minutely 

 punctured (fig. 8,d). As growth proceeded the orifice was drawn 

 out into a tubular vestibule, and the spaces between the aper- 

 tures, keeping pace with the development, were filled by vesi- 

 cular tissue. Just below the surface of mature examples this 

 tissue was itself more or less completely filled by a calcareous 



