GENERA OF CH^TETIDM AND AIONTICULIPORID^. 



125 



Obs. — This singular genus was founded by Mr R. Etheridge, 

 jun., and myself for the reception of a small coral from the 

 Lower Silurian deposits of GIrvan, Ayrshire, to which we gave 



Fig. 43. — Prasopora Grayer, Nich. and Eth., jun. A, Under side of a small example, showing 

 the epitheca, of the natural size. B, Side view of a larger specimen, of the natural size. 

 C, Tangential section enlarged twenty times. D, Vertical section similarly enlarged : 

 a, One of the large corallites ; b, One of the small corallites. The dark shading in the 

 microscopic sections indicates the presence of the matrix. 



the name of Pj'asopoj'a GraycE. The characters of the genus 

 and species have been so fully treated of by us {loc. cit. and 

 Monograph of the Sil. Foss. of Girvan, Fasc. I., pp. 44-48), 

 that it is unnecessary for me to do more than briefly allude to 

 the most striking features of the structure of the corallum. 



In general form and size the corallum of Prasopoi^a GraycB 

 (fig. 43, A and b) precisely resembles that of moderately young 

 examples of Monticulipora petropolitana, Pand., with which a 

 merely external examination would almost certainly lead the 

 observer to place it. This, therefore, affords another example 

 of the uselessness of attempting to decide the true structure 

 and position of any Monticuliporoid by an appeal to its form 

 and general aspect. The corallum is conspicuously and very 

 remarkably dimorphic, the large and small corallites being uni- 

 formerly distributed throughout the entire colony, and being 

 singularly different in internal structure. The structure of the 

 large corallites is most easily recognised in longitudinal sections 

 (fig. 43, D, a), in which each Is seen to possess a central tube, 

 occupying the axis of the visceral chamber, and entirely sur- 



