226 Dr. H. A. Nicholson on some new or 



graphical Society, I have had occasion to study a very exten- 

 sive series of forms belonging to the same group from the 

 Devonian and Silurian formations of the Continent of Europe. 

 Some of these are new, while others are incompletely known, 

 and though I have had the opportunity of figuring some of 

 these in the first part of my Monograph, I have not been able 

 to give any descriptions of them. In the present communica- 

 tion therefore I propose to give brief descriptions, accompanied 

 by figures, of some of the new or imperfectly-known types in 

 question, reserving for a future memoir a number of further 

 forms which similarly require illustration and description. 

 The figures given all represent the microscopic structure of 

 the species described, want of space rendering it impossible to 

 figure the actual specimens from which the microscopic slides 

 were taken. This omission is the less to be regretted as 

 specific and generic distinctions, in the great majority of cases, 

 among the Stromatoporoids are necessarily drawn from the 

 details of the microscopic structure, the general form and mode 

 of growth often being precisely the same in types of the most 

 diverse affinities, while very wide variations in these parti- 

 culars may be found within the limits of a single species. 

 Several of the species described occur in Britain ; but, with 

 one or two exceptions, the figures given are taken from 

 foreign specimens, as I shall have the opportunity of fully 

 illustrating elsewhere the British examples of the same species. 

 I may add that, except in the case of two figures (PI. VI. 

 figs. 6 a and 7 o), the drawings are all on a uniform scale of 

 enlargement, being magnified about twelve times ; and I have 

 been greatly assisted in their preparation by a series of excel- 

 lent photographs taken for me by Mr. George Gellie, of 

 Aberdeen. 



Actinostroma dathratum^ Nich. (PI. VI. figs. 1-3.) 

 Stromatopora concentrica, auctt. 



Coenosteum massive and very irregular in shape, usually 

 spheroidal in form, growing from a small base of attachment, 

 and consisting of numerous successive strata superimposed 

 one upon the other. Kadial pillars stout, usually from ^ to | 

 niillim. apart, the concentric laminee being in general placed 

 at a similar distance apart. The horizontal processes or 

 "arms" are given off from the radial pillars with great regu- 

 larity in radiating whorls, the result being the formation of an 

 angular meshwork, which in tangential sections has a close 

 resemblance to the structure of an hexactinellid sponge. The 

 angular pores, formed as above, served for the emission of the 



