2 Frof. Jl. A. Nicholson on some netc or 



of these do not open on the surface by prominent apertures. 

 As regards minute structure, the coenosteum is made up of 

 closely-set horizontal lamina?, which are only slightly or 

 not at all undulated, but which are minutely crumpled, so as 

 to give rise to more or less imperfect radial pillars. The 

 radial pillars are invariably confined to their respective inter- 

 laminar spaces, and are therefore not " continuous." The 

 interlaminar spaces are broken up into minute, often imper- 

 fect, lenticular cells, which are essentially formed by curved 

 inflections of the concentric laminae. 



In general, from eight to ten interlaminar spaces or from nine 

 to eleven lamina?, occupy the space of 1 millim.* The cells 

 formed by the radial pillars or by the bendings of the laminae 

 are very variable in size, but are mostly from i to ^ millim. 

 in length. 



Ohs. C. vesiculosum is the type species of the genus Cla- 

 throdictyon f, Nich. & Mur. As in all the species of the 

 genus, therefore, the radial elements of the skeleton are incom- 

 pletely developed, and never extend from one interlaminar 

 space into another. While the radial pillars are thus imper- 

 fect, the concentric laminge of the coenosteum are minutely 

 undulated, and the interlaminar spaces thus become broken 

 up and subdivided into vesicles, the size and shape of which 

 are exceedingly variable. Hence vertical sections of C. vesi- 

 culosum (PI. I. figs. 1 and 3) show a minutely vesicular struc- 

 ture, the coenosteum appearing to be made up of minute 

 lenticular cells, arranged in horizontal or slightly curved 



* It should be understood that all such measurements as the above are 

 approximate and not absolute. They are true of one particular specimen ; 

 but even -within the limits of a single slide the number of interlaminar 

 spaces and laminae in a given space is not absolutely constant. Still more 

 is there a want of complete uniformity when we examine slides taken 

 from different examples of the same species. Nevertheless, in spite of a 

 considerable range of individual variability, each species exhibits a suffi- 

 cient constancy in the average distance at which its component laminae 

 are placed apart to render this character available in specific deter- 

 mination. 



t The genus Clathrodictyon is distinguished from Actinostroma {Stro- 

 inatopora, auctt.) by the fact that the radial pillars are restricted to their 

 respective interlaminar spaces, instead of being " continuous." They also 

 do not give out whorls of radiating " arms," and hence we do not see 

 in tangential sections the " hexactinellid " structure so characteristic of 

 species of Actinostroma, though an approach to this is seen in some species 

 of the genus {e. g. in C. regulare, Ros., sp.). The surface in the species 

 of Clathrodictyon is usually smooth or gently undulated and mostly with- 

 out definite prominences or " mamelons ; " these structures are, however, 

 not universally absent, as I formerly supposed (Mon. Brit. Strom, p. 77), 

 but are present, occasionally or constantly, in two species of the genus. 



