330 



PALEONTOLOGY OF ILLINOIS. 



Fig. 9. a, Eschara artemis, d'Orb. b, E. argyrias, d'Orb., from the Cretaceous of 

 France (after d'Orbiguy). c, diagrammatic vertical section of a Cryptostomatous zore- 

 cium showing the "vestibule" at v, the superior hemiseptum at sh, and the inferior 

 hemiseptum at ih. d, outline of the primitive porfion of a Cryptostomatous zooecium, 

 showing the primitive orifice at o, the superior hemiseptum at sh, and the line of the 

 inferior hemiseptum at ih. e, Small portion of zoarium of a typical Cyclostomatous 

 bryozoan, (Idmonea serpens Linn., after Hincks). 



give us an approximate idea of the primitive cell and aperture, 

 but as these features are often of quite uniform construction, 

 we must look to other characters for generic points. These are 

 supplied by the superficial orifice, the presence or absence of 

 mesopores, acanthopores, median tubuli and hemisepta, and by 

 modifications in the form of the zoarium, which, as I have 

 already indicated, are very constant in this sub-order. The 

 bilaminar zoarium is very persistent and, perhaps, the most 

 typical, but the fenestrated forms are more numerous as well as 

 exceedingly constant in their zoarial peculiarities. This is fortu- 

 nate, since the small, yet readily recognized variations in the 

 construction of the zoarium, supply us with excellent points 

 upon which to found our genera. On the other hand the in- 

 dividual zooecia of the fenestrated forms, excepting Pliylloporina, 

 are so uniform that no recognizable peculiarities can be said to 



