﻿STROM ATOPORA TYPICA. 169 



the species, and it likewise occurs at Sotenich. Examples of this variety also 

 occur in the Devonian pebbles of Teignmouth. Lastly, the type which I have 

 provisionally designated 8. concentrica, var. astrigera, appears to be confined to the 

 Devonian Limestones of Devonshire, occurring in the Teignmouth conglomerates, 

 and in the limestone of Chinkenwell Quarry, near Marychurch. , 



2. Stromatopora ttpica, von Rosen. PI. I, fig. 3 ; PI. V, figs. 14 and 15 ; PL XXI, 



figs. 4—11 ; and PI. XXII, figs. 1 and 2. 



Stromatopora ttpica, von Rosen. Ueber die Natur der Stromatoporen, p. 58, 



Taf. i, figs. 1—3, and Taf. ii, fig. 1, 1867. 

 — — Nicholson. Monogr. Brit. Stromatoporoids, General Intro- 



duction, pi. i, fig. 3, and pi. v, figs. 14 and 

 15, 1886 (figured but not described). 



The cosnosteum in this species is typically hemispherical or discoid, more 

 rarely laminar, with a flattened or concave base which is covered by a concen- 

 trically wrinkled epithecal membrane (Plate XXI, figs. 4 and 5), the organism 

 being usually attached to foreign bodies by a limited portion of its lower surface. 

 The size of the coanosteum varies from less than two centimetres up to a foot or 

 more at its base. 



The mode of growth is always by distinct " latilamince," which are not made 

 up of recognisable finer concentric lamina?, and which are always gently curved or 

 bent, the exterior being thus destitute of conspicuous eminences or " mamelons." 

 The surface, in well-preserved examples, shows a minutely vermiculate network 

 (Plate XXI, fig. 7), pierced by innumerable small and close-set circular apertures, 

 representing the mouths of the zooidal tubes. Astrorhizse are always developed 

 in great numbers, but are slightly branched, and are of small size, their centres 

 averaging about 6 mm. apart. They may be superimposed in vertical systems, 

 with a common axial canal to each system ; but this arrangement is rarely distinct, 

 each astrorhiza usually showing two or more small apertures at its centre where 

 it terminates on the surface. 



As regards its internal structure, the skeleton is completely " reticulate," the 

 horizontal elements of the coenosteum ("connecting-arms") being indistinctly 

 developed as separate from the radial pillars. The skeleton-fibre is about \ mm. 

 in diameter, and is minutely porous (Plate XXI, figs. 9 and 10), the network formed 

 by its inosculations being of a close and fine character. Vertical sections 

 (Plate XXII, fig. 2) show that the radial pillars are quite distinct, and are separated 

 by well-developed, approximately vertical zooidal tubes, the cavities of which are 

 intersected by numerous transverse partitions or " tabula?." From six to eight 



