428 On the chief Generic Types of Palceozoic Corals. 



The tabula? of Zaphrentis are usually remarkably well de- 

 veloped, and generally extend right across the entire space 

 occupied by the visceral chamber, from side to side. They 

 very commonly also exhibit a very conspicuous downward 

 curvature near the circumference of the corallum. At other 

 times they become more or less completely blended externally 

 with the dissepiments, which then show a similar downward 

 bending. 



Upon the whole, the genus Zaphrentis, as restricted by us, 

 may be readily recognized by the complete, or comparatively 

 complete, development of the septal system, the great deve- 

 lopment of the tabular, the existence of a fossula, which is 

 formed by the coalescence centrally of a certain number of 

 the septa, and the fact that the dissepiments are in no case 

 sufficiently developed to form an exterior zone of vesicular 

 tissue. Additional characters of more or less importance are 

 to be derived from the characters of the septa, the downward 

 curvature of the tabula? at the outer margins of the corallum, 

 and the total absence of a columella. 



The genus seems to make its first appearance in the Lower 

 Silurian, though much has yet to be done before we can speak 

 positively as to the affinities of some of these ancient forms. 

 It is well represented in the Upper Silurian, and may be con- 

 sidered as attaining its maximum in the succeeding period of 

 the Devonian. It is also largely represented in the Carboni- 

 ferous period, after which it appears to have died out. Z. pa- 

 tula : Mich., and Z. Enniskilleni, Edw. & H., may be taken as 

 exhibiting the typical structure of this genus in its greatest 

 perfection. 



In the Carboniferous deposits of Scotland, the species of 

 Zaphrentis appear to attain their greatest development in the 

 earlier portion of the Mountain-Limestone series. In the 

 upper portions of the same series they appear to be already 

 approaching extinction, and are usually much dwarfed and 

 stunted in growth. 



EXPLANATION OF PLATE XII. 



Fig. 1. Amplexus coralloides, Sow. ; 1 a, transverse section of the same; 

 1 b, longitudinal section of the same. 



Fig. 2. Amplexus nodidcsiis, Phill. ; 2 a, transverse section of the same. 



Fig. 3. Transverse section of another individual of the same; 3 a, longi- 

 tudinal section of the same. 



Fig. 4. Amplexus, sp. ; 4 A, transverse section of the same, cut somewhat 

 obliquely; 4 b, longitudinal section of the same. 



Fig. 5. Transverse section of Zaphrentis Fnnisldlleni, E. & II. The sec- 

 tion is cut above the floor of the calice, and consequently shows 

 no interseptal dissepiments. 



