504 PALAEONTOLOGY OF ILLINOIS. 



turn. Soon after bending from 8 to 10 ridges are developed on 

 the inner surface of the zocecial wall which increase in strength 

 toward the surface. In tangential sections they appear as sub- 

 regular tooth-like projections into the zooecial cavity. Inter- 

 zooecial space filled with layers of lenticular vesicles, of angular 

 form in tangential sections, which are only partially filled with 

 solid tissue near the surface. Those resting upon the mesial 

 laminae a little larger than the others. 



Eominger states that the species sometimes occurs in unilam- 

 inar expansions with an epitheca upon the lower side. I very 

 much doubt the accuracy of this statement, since none of nearly 

 one hundred examples seen by me; are in that condition. 



This interesting and peculiar species is the only one known to 

 me having the radial denticles so greatly developed that they 

 form internal septal ridges. Spinose peristomes are known 

 among the FISTULIPORID^E, and radial denticles in Fenestella, 

 Polypora and Pinnatopora, but these are all very superficial 

 when compared to the radial structures found in A. peculiars. 

 Still, I regard them all as homologous, and with slight modifi- 

 cations, accounted for by one explanation; Namely, I believe 

 that in Actinotrypa, these structures were originally represented 

 by small spines or tubercles on the peristome of the original 

 aperture, which acted as support to converging setae, and keep- 

 ing pace with the gradual elongation or growth of the aper- 

 ture into a tubular "vestibule," they eventually became septa- 

 like ridges. 



Possibly the genus has nearer relatives among the FISTULI- 

 PORiDjE, but the essential points of structure, as I now see them 

 seem to correspond more nearly with those of the CYSTODICTYO- 



NID^E. 



Position and locality: Keokuk group; Keokuk, Iowa; La- 

 Grange, Mo., Warsaw and Nauvoo, 111. 



TJENIOPORA Nicholson, 1874. 



(Geol. Mag. N. S. Vol. I, p. 121.) 

 (For generic diagnosis see page 386.) 



