122 



COELENTERATA— HYDROZOA 



PHYLUM II 





the above-named groixps, but whose exact position in the zoological system 

 remains as yet uncertain. During the Paleozoic era, to which they are con- 

 fined, the Stromatoporoids were important geological agents, whole beds of 

 limestone being often essentially constituted of their remains. In the Mesozoic 

 era they are replaced by very closely allied forms of Hydmctinia, which in all 

 probability represent their immediate descendants. 



The Stromatoporoids secrete hemispherical, globular, nodular or hori- 

 zontally expanded skeletons, which are sometimes encrusting, sometimes 

 attached by a short basal peduncle, and are covered on the under side with 

 concentrically wrinkled epitheca, while the apertures for the emission of the 

 polyps are situated on the upper surface. The general tissue of the coenosteum 

 is composed of numerous, concentric, undulating, calcareous layers or laminae, 

 which are separated by narrower or wider interlaminar spaces, but are at the 

 same time connected by numerous vertical rods {radial pillars). The pillars 

 as well as the laminae are traversed, as a rule, by minute, irregularly directed 



^ canaliculi. In some 



genera the coenosteum 

 is provided with verti- 

 cal tabulate tubes, 

 which most probably 

 served for the recep- 

 tion of the polyps, as 

 in the genus Millepora ; 

 but in many instances 

 they are wanting. The 

 surfaces of the laminae 

 typically exhibit pores 

 and small tubercles, 

 and frequently also 

 shallow stellate furrows 

 (astrorhizae), which radiate outwards from numerous centres. Sometimes the 

 laminae consist merely of a loose network of horizontal calcareous fibres. 



Groldfuss at first held the Stromatoporoids occurring so profusely in the 

 Eifel for corals (Blillepora), and subsequently for sponge-like zoophytes ; while 

 von Rosen considered them as horny sponges that had become secondarily 

 calcified. Sandberger and F. Roemer assigned them to the Bryozoans ; 

 Dawson to the Foraminifera ; Sollas to the siliceous sponges (Hexadinellida) ; 

 and Salter to the calcareous sponges, whose example Nicholson also followed. 

 Lindstrom, Carter and Steinmann subsequently pointed out their relations 

 to Hijdracfinia and Millepora ; and Nicholson and Murie came finally to 

 regard them as a group of extinct Hydrozoans allied to Hydradinia on the 

 one hand (Actinostromidae), and Millepora on the other (Stromatoporidae 

 and Idiosti'omidae). 



Adinostroma Nich. (Fig. 189). Skeleton having vertical or radial pillars 

 disposed at tolerably regular intervals, and extending continuously through all 



Gotland uiid Osel. K. Svensk. Vetensk. Ak.id. Handl., 1908, vol. x\ui. ^Giirich, (?., Les Spongio- 

 stromides du Vise daiisle Province de Namur. Mem. Miis. Ro}'. d'Hist. Nat. Belg. , 1906, vol. iii. 

 — Idem, Neues Jahrb. f. Mineral., 1907, i. — Parana, C. F., La Fauna coralligena del Cretaceo del 

 Monti d' Ocre nell' Abruzzo Aquilano. Mem. Comni. Geol. Ital., 1909, vol. v. —Parks, W.iA., The 

 Stromatoporoids of the Gnel])h Formation in Ontario ; the Niagara ; tlie Silurian of America ; the 

 Ordovitian. Univ. of Toronto Studies, Nos. 4-7, 1907-1910. 





Fig. 1S9. 



Actinostrnma interfextum Nich. Silurian (Wenlock) ; Shropshire. A, 

 Tangential section showing radial pillars and reticulated structure of con- 

 centric laminae. 7i, Vertical section, showing formation of laminae from 

 processes given off horizontally hy radial pillars, I2yj (after Nicholson). 



