426 Mr. J. Thomson and Dr. H. A. Nicholson on the 



we are not prepared to assert positively that this character is 

 distinctive of the genus. 



The calice is approximately circular in form, with a thin 

 upper margin, and of moderate depth. ' Owing to the short- 

 ness of the septa, the bottom of the calice is nearly flat, and is 

 formed by the upper surface of the first tabula. 



Upon the whole the genus Amplexus must be regarded as 

 a very distinct one, the type of which is found in A. coral- 

 hides, Sow., of the Carboniferous. The earliest examples of 

 the genus with which we are acquainted appear in the Upper 

 Silurian ; and the group undergoes a considerable development 

 in the Devonian, where it is represented by such typical forms 

 as A, Yandetti, Edw. & H., and A. tortuosits, Phill. It is, 

 however, in the Lower Carboniferous that the headquarters 

 of the genus is found, and where it obtains its greatest deve- 

 lopment both as regards the number of species and the size of 

 individual examples. 



Genus Zaphrentis. 



Zaphrentis, Rafinesque and Clifford, Ann. des Sci. Phys. de Bruxelles, 

 vol. v. p. 234. 



Gen. char. Corallum simple, turbinate, conical or cylindro- 

 conical, usually more or less curved. Calice deep, with a 

 large and conspicuous fossula, the position of which is variable, 

 but which is seen on transverse section to be formed by the 

 coalescence of a portion of the septa, which fold round and 

 form its walls. Septa well developed, extending nearly or 

 quite to the centre of the visceral chamber. No columella is 

 developed. The tabulae, though varying in their development, 

 are always a conspicuous feature ; and the septa are prolonged 

 over their upper surfaces. The interseptal loculi are usually 

 filled up towards the circumference of the corallum by convex 

 dissepiments ; but these are generally more or less remote 

 and irregular, and the tabula? are continued through them to 

 the wall. 



The general form of the corals belonging to Zaphrentis is 

 turbinate or trochoid ; but they sometimes assume a more or 

 less cylindrical form, and may attain a very great size. Other 

 species, again, are singularly small in point of size. The 

 epitheca is thin, and is usually marked with fine encircling 

 striae, whilst accretion-ridges are not uncommonly present. 

 The calice is deep, with attenuated margins ; and there is 

 always a large and deep fossula. The position of the fossula 

 varies, being sometimes on the convex or dorsal side of the 

 corallum, sometimes, lateral, and commonly on the concave or 



