SYNOPSIS OF THE GENERA INCLUDED IN THIS VOLUME. 



Streptelasma, HaU (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. Tabulas 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 stridum, 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 platesnumer- 

 OU.S, 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 ahso 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 Germaniae, 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 



