Species of Poly zoa from the Devonian Rocks. 79 



of a Retepora. It would thus appear that the water must have 

 been admitted to this central space, and thus to tlie cells, by 

 openings in the free edge or lip of the infundibuliform poly- 

 zoary ; but none of my specimens exhibits this portion of the 

 frond. 



In the genus Hemitrypa.^ Phillips, the fenestrules do not 

 perforate the coenoecium so as to reach the outer face of the 

 frond, but are filled by a calcareous membrane. The cells, 

 however, open upon the external surface, instead of into a 

 central space ; and the structure of the polyzoary is in other 

 respects very different. 



The following is the only species of the genus that I have 

 as yet met with, though additional forms will probably be 

 afterwards detected. 



Cryptopora mirabilis (Nicholson). 



Polyzoary infundibuliform, apparently from one to three 

 inches in height and three quarters of an inch or more in 

 diameter distally. External layer thin, imperforate, smooth 

 or obscurely striated. Intermediate layer formed of the coa- 

 lescent branches, and marked externally by vertical shallow 

 grooves, which are placed about half a line apart and some- 

 times bifurcate. Between these grooves the external surface 

 is mapped out by inosculating lines into small oval or poly- 

 gonal s])aces corresponding with the cells. The internal 

 surface of the intermediate layer carries the cells, which are 

 arranged biserially in flexuous lines, and enclose oval or 

 rhomboidal interspaces. These interspaces are arranged in 

 very regular diagonal lines, about four in the space of two 

 lines ; and they give origin to a series of short rounded pillars, 

 which extend inwards at right angles to meet the internal 

 layer. The central space, in which the cells are situated, is 

 about half a line to two thirds of a line in depth. The internal 

 layer is apparently thin and membranous. The entire frond 

 springs from an exceedingly strong, horizontal, branched 

 stalk, the surface of Avhich is marked by vermicular stria;. 



The materials in my hands are not sufficient to permit of an 

 entirely satisfactory elucidation of the characters and structure 

 of this remarkable species. Different specimens, however, or 

 different parts of the same specimen, show the following 

 appearances : — 



1. The external membrane is thin, and is only preserved 

 in parts of any specimen that I have seen ; and as it corre- 

 sponds with the reverse or non-poriferous face of an ordinary 

 Retepora^ it is to be regarded in reality as nothing more than 

 the exterior portion of the intermediate layer. 



7* 



