CLA1TIR0DICTY0N FAST1GIATUM. 153 



thickness in the centre of from 2 to o cm. The under surface (Plate XIX, fig. 2) 

 is covered with a concentrically-wrinkled epitheca. The superior side of the 

 ccenosteum is flat, or slightly undulated (Plate XIX, fig. 1), but is quite free from 

 " mamelons." The surface exhibits, when well preserved, numerous vermiculate 

 and inosculating ridges formed by rows of elongated tubercles (Plate XIX, fig. 3). 

 Small and remote astrorhiza3 may sometimes be recognised in thin sections ; but 

 their development is imperfect, and I have not detected their presence on the free 

 surface. 



As regards its internal structure, the coenosteum is composed of bent and 

 crumpled concentric lamina), of which about five (or four interlaminar spaces) 

 usually occupy the space of 1 mm. As shown by vertical sections (Plate 

 XIX, fig. 5) the laminae are bent in two ways. In the first place they are bent 

 into numerous chevron-like foldings, no traces of which appear on the surface 

 of the ccenosteum. In the second place each lamina is minutely crumpled or 

 inflected in such a way that the interlaminar spaces are constricted into rows of 

 very imperfect and more or less open vesicles. The radial pillars are developed 

 from the point of inflection of the laminae, but are thin and largely imperfect. 

 Hence, in vertical sections, the bent and crumpled lamina? are far more conspicuous 

 than the radial pillars. Tangential sections (Plate XIX, fig. 4) exhibit the 

 irregularly sinuous and vermiculate edges of the transversely-divided and folded 

 laminae, the cut ends of the radial pillars appearing in these as dark rounded dots. 

 Occasionally we may also recognise in tangential sections scattered points round 

 which rows of dots are disposed in a radiating manner. Such points represent 

 the centres of small astrorkizae. 



Obs. — 0. fastigiatum has certain relationships with G. variolate, Rosen sp., and 

 specimens occasionally occur which present a mixture of the characters of the two 

 forms. In typical examples, however, the present beautiful species cannot readily 

 be confounded with any other known member of the genus Glathrodietyon. It is 

 distinguished from its nearest allies (viz. G. variolare, Rosen, and G. vesiculosum, 

 Nich. and Mur.) by the greater remoteness of the concentric laminae, and by the 

 peculiar and constant chevron-like and angular folds into which the laminae are 

 thrown. The appearances presented by tangential sections are also exceedingly 

 characteristic, and quite unlike those seen in any other species of Glathrodietyon 

 with which I am acquainted. The very imperfect development of the astrorhizae is 

 also a point in which the present species is separated from the forms above alluded 

 to. Lastly, as far as I have seen, the ccenosteum of G. fastigiatum always has 

 the form of a thin, cake-like expansion, with a concentrically wrinkled epitheca 

 below. 



Distribution. — G. fastigiatum occurs abundantly in the Wenlock Limestone of 

 Britain, and I have specimens of it from Ironbridge, Dudley, Much Wenlock, and 



