CLATHRODICTYON VESICULOSUM. 149 



all these forms, and examples are not uncommon which show intermediate or 

 transitional characters, and which, therefore, it is difficult to refer definitely to 

 any one of the four. In a less striking form, a passage may also be traced between 

 this group of types and G. fastigiatum, Nich. 



From all of the allied forms above mentioned G. vesiculosum is best distin- 

 guished by the extreme closeness with which the concentric laminse are set, and 

 the resulting minuteness of the cells which compose the coenosteum. Not only are 

 the laminse exceedingly close, but the interlaminar spaces are nearly equal in 

 width, and the cellular tissue of the skeleton is thus approximately uniform. On 

 the other hand, in typical examples of C. variolar c, Rosen sp., the interlaminar 

 spaces are unequal in size — wide spaces alternating tolerably regularly with spaces 

 which are much narrower than the average, and which aro usually arranged in 

 groups (Plate XVIII, fig. 1). Trivial as this difference is, it is one which gives a 

 very different aspect to vertical sections of these two forms. In other examples 

 of G. variolare, in which this alternation of wide and narrow interlaminar spaces 

 is not so marked, all the interlaminar spaces are wider and the cellular structure 

 is coarser than in the present species. In G. Linnarssoni, Nich., again, the 

 concentric laminse are much less crumpled and the cells of the interlaminar spaces 

 are, therefore, much more nearly quadrangular than is the case in G. vesiculosum. 

 The form which I have named G. crassum is sufficiently distinguished from the 

 present species — when characteristic examples of the two are compared with one 

 another — by the comparatively small number of concentric laminae in a given 

 space, and by the much thicker and coarser structure of the skeleton-fibre. Lastly, 

 G. fastigiatum, Nich., is in general readily separated from G. vesiculosum by the 

 much greater width of the interlaminar spaces and the chevron-like foldings 

 of the concentric laminse. 



Distribution. — Clathrodictyon vesiculosum, so far as known, is exclusively 

 confined to the Silurian Rocks, and is especially characteristic of the lower portion 

 of the Silurian (May Hill and Wenlock zones). In strata of this age the species 

 has, in fact, an extraordinarily wide range. In England G. vesiculosum is of 

 comparatively common occurrence in the Wenlock Limestone, and I have 

 collected examples of it at Much Wenlock, Dudley, Ironbridge, and Dormington. 

 In Scotland it has been collected by Mrs. Robert Gray in the Silurian Rocks 

 (Woodland-beds) of Woodland Point, near Girvan. In North America the species 

 seems to be abundant in the Clinton and Niagara formations, and I have personally 

 collected examples in the Clinton-beds of Yellowsprings, Ohio, and the Niagara 

 Limestone of Canada (Thorold). By the kindness of Mr. Whiteaves I have also 

 recently had submitted to me examples of this species collected by the Geological 

 Survey of Canada in the Clinton formation of Anticosti (Junction Cliff and west 

 side of Gamache Bay). I have, further, collected examples of this species in the 



