188 BRITISH STROMATOPOROIDS. 



and the digitated variety occur commonly in the Middle Devonian Limestones of 

 Dartington (Pit-Park Quarry), and are found abundantly in the Devonian pebbles 

 of the Triassic conglomerates at Teignmouth. In Germany the species is not 

 uncommon in the Middle Devonian Limestone of Biichel, in the Paffrath district, 

 most examples from this locality being lobate in form. The species does not seem 

 to occur at Hebborn, in the same district, and is of rare occurrence at Gerolstein, 

 in the Eifel. 



9. Stromatopora discoidea, Lonsd. sp. PI. Ill, fig. 3 ; PI. VII, figs. 1 and 2 ; and 



PL XXIV, figs. 2—8. 



Pobites discoidea, Lonsdale. Silurian System, p. 6S8, pi. xvi, fig. 1, 1839. 

 Heliolites? discoideus, Salter. In Murchison's Siluria, 3rd ed., pi. xxxix, 



fig. 1, 1859. 

 Steomatopoea elegans, von Rosen. Ueber die Natur der Stromatoporen, p. 63, 



Taf. ii, fig. 8, and Taf. iii, figs. 1 and 2, 

 1867. (Non Stromatopora elegans, Carter, 

 Ann. and Mag. Nat. Hist., ser. 5, vol. iv, 

 1879.) 

 Ccenosteoma DiscoiDEUM, Lindstrmn. Kongl. Svenska Vetenskaps-Akad. Haiid- 



lingar, Bd. ix, Taf. i, figs. 6 and 7, 

 1870. 

 Steomatopoea discoidea, Nicholson. Monogr. Brit. Strom., General Introduction, 



pi. iii, fig. 3, and pi. vii, figs. 1 and 2, 1886 

 (figured but not described). 



The ccenosteum of this species is essentially of the laminar or expanded type, 

 and usually has the form of a thinner or thicker disc, attached by a portion of the 

 base to some foreign body, and having the rest of the under surface covered with 

 a concentrically wrinkled or striated epithecal membrane. In some cases, while 

 the same general form is retained, the ccenosteum is so thickened as to assume a 

 hemispherical shape, the base remaining flat or concave. In the thinner examples 

 growth of the skeleton is continuous, but in the thicker examples more or less 

 regular " latilaminge " are observable, indicating periodic pauses in the process of 

 growth. 



The concentric laminae are simply undulated, and the surface is therefore free 

 from eminences or " mamelons." Astrorhizse are extraordinarily developed, 

 being not only very numerous, but being of an excessively ramified or arborescent 

 type (Plate III, fig. 3 ; and Plate XXIV, fig. 2). The astrorhizae vary in size in 

 different individuals, their centres being usually from 5 to 7 mm. apart ; but in all 

 cases they become more or less completely confluent with one another by the 



