136 Prof. W. King on Pleurodictyum problematicum. 



curving over, by, or below it. Both modifications are exhibited 

 in PI. X. figs. 1, 2, 4 & 9. 



With respect to the cell-apertures, as exhibited on the upper 

 or convex surface, I have endeavoured by grinding down a 

 few casts, from their basal or concave surface, to exhibit them. 

 The specimens at my service for this purpose are, however, so 

 few, that I have been compelled to represent a gutta-percha im- 

 pression taken from a very indiiFerent one (vide PL X. fig. 10). 

 The apertures it will be seen are somewhat circular and sub- 

 polygonal; and the central portion contains the apertures of 

 several small cells, the size of which is attributable to the ver- 

 miform appendage lying beneath them, thereby preventing their 

 full development. 1 failed in discovering the terminations of the 

 appendage in this specimen; but judging from others, I am 

 disposed to place them near the marks * *. 



Pleurodictyum having been generally considered a coral, it 

 may be expected that 1 should examine the grounds on which 

 this opinion is founded ; but I contend, that it is quite unne- 

 cessary for me to enter on such a step : all I have to perform is 

 to prove that the vermiform appendage is integral to the fossil. 

 It may also be observed, that even before the generally received 

 view can be discussed, the appendage must first be proved to be 

 a foreign body. Were it not for the vermiform appendage, I 

 should at once subscribe to the opinion that our fossil was a 

 coral ; — the only question with me would be, as to w^hat division 

 of the class Corallaria it belonged. 



I am quite willing, however, to institute a comparison between 

 Pleurodictyum and those corals with which it has been associated, 

 a step which may be of some service in working out the question 

 shortly to be discussed. 



The family Poritidce, in which MM. Edwards and Haime 

 place our fossil, is composed of genera, having, with a few 

 tertiary and Jurassic exceptions*, all its representatives living in 

 our present seas. The agreement between it and the genera 

 alluded to consists in the foraminated character of the cell-walls 

 and the absence of horizontal plates : but there is an important 

 difi*erence prevailing; as in the group named a well-developed 

 septal apparatus (consisting of vertical plates) exists, which is 

 absent in the fossil f. 



* Milne-Edwards and Jules Haime, since the Introduction to their 

 Monograph of British Fossil Corals was written, have added a new 

 genus to the family Poritidce under the name Protarcea. It is typified 

 with Hall's Silurian Poriies vetusta. This species and another {Protarcea 

 Verneuili) occur in the Silurian beds of North America. 



t Mr. Rupert Jones has kindly supplied me with a copy of Edwards and 

 Haime's diagnosis of Pleurodictyum since much of this paper was written. 

 As I differ from these able Invertebratologists respecting the homology of 



