450 PALAEONTOLOGY OF ILLINOIS. 



appear in tangential sections at the angles of the zooecia which 

 simulate acanthopores, but are not of that nature so far as I 

 can make out. 



The solidity of the stems is the character that separates this 

 form readily from A. symmetries,, the type of the genus, figured 

 on PI. LXXII, fig. 5; it also lacks the single very large aper- 

 ture at the centre of the clusters, which is such a characteristic 

 feature of that species. The much thicker interspaces distin- 

 guish the species from A. ramulosa. 



Position and locality: Chester group. Sloan's Valley, Pu- 

 laski Co., Ky., and Chester, 111. 



AMPLEXOPORA. Ulrich, 1882. 



("Am. Pal. Bry." Jour. Cin. Soc. Nat. Hist. Vol. V. p. 154.) 

 (For generic diagnosis see page 377.) 



Fourteen species of this genus, ranging in time from the 

 Trenton to the Hamilton group, have been studied by me. 

 The two here described have thinner walls than is usual in the 

 genus, differing in that respect conspicuously from A. cingulata. 

 Still they present all the essential characters of Amplexopora,, 

 and are brought into closer connection with the typical species 

 than at first appears by such intermediate forms as A. superba 

 Foord, A. septosa and A. robusta Ulrich. In their general 

 structure species of Amplexopom resemble ramose species of 

 Stenopora Lonsdale, but are fundamentally distinguished by 

 the opercular structures, perforated diaphragms, and periodi- 

 cally constricted zocecial walls of that genus. 



AMPLEXOPORA AFFINIS Ulrich. 



PL XXXVI. fie. 2-2a. 



Zoarium irregularly dendroidal, branching seldom, about fif- 

 teen mm. in thickness. Surface smooth, exhibiting scarcely ap- 

 preciable clusters of cells slightly larger than the average. Cell 

 apertures subpolygonal. Zooecial tubes polygonal, thin-walled, 

 seven or eight in two mm. Diaphragms somewhat more than a 

 tube diameter apart in the axial region, in the peripheral re- 

 gion about twice as close. A few infundibular diaphragms scat- 

 tered about promiscuously. Acanthopores conspicuous, usually 



