SYNOPSIS OF THE GENERA INCLUDED IN THIS VOLUME. 
Streptelasma, Hall (Palaeontology of N. Y.,vol. i, p. 17. 1847). 
[Type, Streptelasma expansum, Hall.] 
Corallum simple. Lamellae usually simple, alternate, coalescing in the cen¬ 
ter, forming a vesiculose core or pseudo-columella. Tabulae and fossette 
obscure or obsolete. 
The species grouped under this genus, from various geological horizons, show 
considerable variation in form and character, and pass gradually into the genus 
Zaphrentis, of which they may be considered as a sub-division or sub-genus. 
Example: Streptelasma strictum, pi. i, figs. 1-10. 
Zaphrentis, Rafinesque (Ann. des Sci. Phys. Brux., vol. v. 1820). 
[Type, Zaphrentis phrygia, Rafinesque.] 
Corallum simple, conical. Lamellae simple, alternate, rarely extending to the 
center; usually arranged in four fascicles, with a well marked septal fossette be¬ 
tween the two anterior fascicles. Diaphragms or transverse laminar plates numer¬ 
ous, extending from wall to wall and deflected downwards around the periphery. 
The species illustrated and described in this volume, from the Lower Helder- 
berg group, offers a very good illustration of the uncertainty of many of the 
characters which are often considered as of specific importance. The fossette 
is variously developed, and is not constant in its position in the cup. The 
lamellae are also subject to much variation in their number and extent; some¬ 
times they extend quite to the center and become irregular and involved; in 
other examples again they extend not more than one-third the radius of the 
cup, and expose the central area of the diaphragm. 
Example: Zaphrentis Roemeri, pi. i, figs. 11-21. 
Aulopora, Goldfuss (Petrefacta Germanise, vol. i, p. 82. 1826). 
[Type, Aulopora serpens, Goldfuss.] 
Colonies of procumbent tubes, usually attached for the greater portion of 
their length. Tubes increasing by latero-basal gemmation; after budding each 
