146 Br. Alex. Brown — On Soknopora. 



In a monograph on Solenopora compacta (Geol. Mag. Dec. III. 

 Vol. II. p. 529), Professor Nicholson refers the genus Solenopora 

 temporarily to the Hydrozoa ; but he says that " if evidence can 

 be obtained, proving decisively the existence of a cellular, and not 

 a tubular, structure in Solenopora, then the reference of the genus to 

 the calcareous Algse will follow as a matter of course." Now, from 

 recent investigations carried on upon all the forms enumerated, and 

 from a comparison of structure of these with those of Tertiary and 

 recent Corallinese, I have been led to the decisive conclusion that 

 the genus Solenopora is in reality an ancient form of the same gi'eat 

 group, to which our existing Corallina, Amphiroa, Melobesia, and 

 Lithothamnion belong. Before discussing this subject in detail, how- 

 ever, it will be advisable that I should give a brief description of 

 the various species of Solenopiora with which I am at present 

 acquainted. 



I. Description of Species. 

 1. Solenopora compacta, Billings sp. 



Stromatopora compacta, Billings. " Palaeozoic Fossils" (1861-65). 



Tetradium Feachii, Nicholson and Etheridge, jun. Ann. and Mag. Nat. Hist. 



ser. iv. vol. xx. p. 166 (1877). 

 Solenopora spongioides, Dybowski. "Die Chaetetiden der Osbalt. Silur. Form." 



p. 124, taf. ii. figs. Ua, b (1877). 

 Tetradium Feachii, var. Canadense, Foord. Contrib. Micro. Pal. Sil. Eocks, Canada, 



Gaol. Survey, 1883, p. 24. 

 Solenopora compacta, Nicholson and E. Etheridge, jun. Geol. Mag. Dec. III. 



Vol. II. No. 12, p. 529. Dec. 1885. 

 Solenopora compacta, Nicholson. Geol. Mag. Dec. III. Vol. V. No. 1, p. 15, 

 Jan. 1888. 



" This species occurs in subspheroidal masses, from the size of a 

 hemp-seed up to the dimensions of an orange. External surface 

 lobulate. Fractured surface has a porcellanous, sometimes fibrous 

 aspect, and is usually white or light-brown in colour. The fractured 

 surface also shows signs of a more or less obvious composition out of 

 concentrically-disposed strata " (Nicholson). 



In vertical section the cells are elongated, arranged in a radiating 

 and parallel fashion, and are often disposed in concentric layers. 

 Diameter of cells about -^j- mm. The cells have distinctly undulating 

 walls (Fig. 1). 



In tangential section the cell-walls appear sinuous, and in many 

 cases traces of active cell-division are observed. Certain specimens 

 show a distinct variation in the size and form of the cells. In these 

 two sets of cells are to be noted — one set small, with a wavy or 

 polygonal outline; the other set large, elongated or oval in form, 

 and scattered irregularly amongst the smaller cells. The large cells 

 represent the spore-producing cells (sporangia), and the small the 

 ordinary tissue-cells of the organism (Fig. 2). 



Varieties. — There exist certain varieties of the above species which 

 differ from the type-form, either in the size of the tissue-cells, or 

 in the structure and form of the cell-walls. 



Solenopora compacta, var. Trentonensis, nov. var. This variety, 

 illustrated in Fig. 2, is found in the Trenton Limestones, and varies 



