140 LOWER PENINSULA. 



tersected by the diaphragms, and are divided into coarse cellulose 

 chambers, which must not be confused with the smaller interstitial 

 cell spaces of the outer area in a Cyathophyllum. The latter are 

 formed by rows of small vesiculose plates traversing the inter- 

 stices in arches, ascending from the inner circumference of the dia- 

 phragms to the outer walls of the polyparium, within the end cup 

 as well as below it. The transverse interstitial plates which make 

 part of the diaphragms invariably approach the outer walls in a 

 downwardly instead of an upwardly directed curve. The arrange- 

 ment of the lamellae into four fascicles is, in Zaphrentis, generally 

 well marked, and a largely developed septal fovea in the interstice 

 between the two apertural fascicles is considered as the leading dis- 

 tinctive character of the genus Zaphrentis. 



The importance of the development of a septal fovea in Cyatho- 

 phylloid corals has been unreasonably overestimated by Milne- 

 Edwards. In his work on palaeozoic corals, he describes twenty- 

 nine species of Zaphrentis, simply guided by the presence of a large 

 septal fovea, in disregard of other structural peculiarities. Fully 

 half of these forms, named Zaphrentis, have not the structure of 

 Zaphrentis, and belong to the true Cyathophylla, by reason of the 

 development of transverse vesiculose plates in the interlamellar 

 interstices of the end cups, which, as has been stated before, never 

 occur in the interstices of the lamellae of a Zaphrentis. As ex- 

 amples of Cyathophylla, so misplaced, I mention Zaphrentis 

 cornicula, Zaphr. Guerangeri, Zaphr. excavata, Zaphr. Michelini, 

 etc. The genus Streptelasvia, of Hall, and his Polydilasma, which 

 1 consider as the same thing, differ from Zaphrentis- merely in hav- 

 ing a less conspicuous, or, sometimes, almost obsolete develop- 

 ment of a septal fovea, and in the irregular entanglement of its 

 radial lamellae in the centre, constituting, in combination with the 

 intersected diaphragms, a spongioso-cellulose pseudo-columella ; or, 

 at least, the regularity of the diaphragms in the centre is obscured 

 by their multiple labyrinthical intersection with the vertical leaves. 

 A strict distinction of the two genera is not possible ; some forms 

 of this group could with the same propriety be placed in one as 

 in the other. 



The Cyathophylloid corals of the lower Silurian strata generally 

 have the structure of Streptelasma, and, considering Streptelasma 



