OF CENTRAL CANADA PART IV. 



231 



preserved examples is not always observable. Its presence is usually 

 marked by a ridge upon the outside of the cup or cell. 



Whether radiating septa are present or absent, the interior of the 

 cell in many genera typically, in all the HYDROCORALLA is divided 

 transversely by a series of plates or " tabulae," also called " dia- 

 phragms," which extend more or less regularly across the cell, and are 

 flat or arched in form (Fig 135 a. b.). In other cases, these tabulae, 

 when present, are confined to the more central part of the cup, or cell, 

 the sides of the latter being filled with short, irregular, plates, techni- 

 cally known as " vescicular tissue" (Fig. 135c). Occasionally, 

 again, as in the genus Cystiphyllum, this vesicular tissue occupies the 

 entire cell (Fig. 135c?). 



Viewed broadly, the HYDROCORALLA* may be classed under six 

 sections, as in the following table : 



1. Inornata. 



2. Tabulo-Stellata. 



3. Vesiculo-Stellata. 



4. Vesiculosa. 



5. Operculata. 



6. Integri-Stellata. 

 These sections are defined as follows : 



1. Inornata : Tabulae well developed, extending entirely across 

 the cell. Radiating septa absent, or quite rudimentary. 



All the genera of this section are compound forms ; and all, with 

 the exception of Millepora (a living type, dating only from the 

 Eocene period) are extinct Palaeozoic types. The more common Cana- 

 dian genera comprise : (1) Fistulipora, mostly in irregular, encrust- 

 ing masses with very small cells of two sizes, the smaller forming a 

 kind of pseudo coenenchyme. (2) Monticulipora (including in part, 

 Stenopora and Ghcetetes), with narrow, capilliform cell-tubes, in 

 branching and rounded or hemispherical examples, Fig. 130. (3) 

 Favosites, in irregular and pyriform, sometimes branching, masses, 

 composed of polygonal cell- tubes with perforated walls and straight 

 tabulae, Fig. 137. (4) Alveolites Fig. 137 a. with obliquely-opening 

 cell-mouths and perforated walls : (5) Michelinea. with short, wide 



*The subdivision of the Hydrocoralla, as here adopted, may be thus defined: Hydrozoa or 

 allied types with internal calcareous corallum : the latter containing <1) tabulae, with or with- 

 out radiating septa ; or (2), vesicular tissue, with or without tabulae and septa; or (3), a dis- 

 tinctly tetramerous system of septa ; or (4), indications of bilateral symmetry. 



