388 Dr. H. A. Nicholson and Mr. K. Etlieridge on 



tD^ 



as we do, ojie perfect symmetrical combined growth, we ouglit 

 to have exhibited to us two independent symmetries, as occurs 

 in true cases of commensalism. 



But I will suppose, as I believe to be the case, and as was 

 believed unreservedly in early times by botanists generally, 

 that the lichen is a single autonomous organism, and not a 

 compound of two distinct species of plants. I would look 

 upon the more or less developed sporidial tissue as the repro- 

 ductive portion, and the gonidia as the vegetative portion of 

 this lichen-structure ; and I feel scarcely a difficulty in be- 

 lieving that the mutual organic union of the two is necessary 

 to constitute the complete autonoi^ous lichen capable of pro- 

 ducing sporidia. It is true that this procedure of development 

 of a simple species of plant is ajDparently exceptional and 

 somewhat anomalous ; but the acceptance of this view gets 

 rid of the paradoxes necessarily attaching, in the case in ques- 

 tion, to that theory of parasitism which has been strongly 

 contended for by some celebrated botanists. 



We must not fail to bear in mind that in other families of 

 the lower plants there have been encountered unexpected 

 phenomena connected with tlieir reproduction ; and it is only 

 necessary to mention the Filices as furnishing a notable in- 

 stance of such an unlooked-for and startling discovery. 



The fact that there are some lichens parasitic upon other 

 lichens is only analogous to Avhat occurs amongst many other 

 families of plants. 



Ceylon, Sept. 10, 1877. 



L. — Contributions to Micro-Palceontohgy . — II. On Prasopora 

 Grayffi, a new Genus and Sjjecies of Silurian Corals. By 

 H. Alleyxe Nicholson, M.D., D.Sc, F.E.S.E., and 

 EoBERT Etheridge, Juu., F.G.S. 



Genus Peasopoea, Nich. & Eth., jun. 



Gen. char. Corallum compound, forming small hemispheric 

 or concavo-convex masses, composed of numerous prismatic 

 corallites radiating from a wrinkled basal epitheca, and con- 

 nected by smaller coenenchymal tubules. Corallites with 

 imperforate walls, destitute of septa, furnished with an exte- 

 rior zone of vesicular tabulaj surrounding a vacant central 

 tube, which may be crossed here and there iiy an occasional 

 tabula. Calices showing a central aperture surrounded by a 

 ring-like tabula. Coanenchymal tubes much smaller than the 



