HYDROZOA. STROMATOPOROIDEA 73 



definite tubes, but in others tubes are absent and there are 

 pores only in the external lamina. From the structure of 

 the skeleton, the Stromatoporoids seem to be connected 

 with both Hydr -actinia and the Hydrocorallina. 



• The Stromatoporoids are found mainly in the Ordo- 

 vician, Silurian, and Devonian Systems, being most 

 abundant in the last ; frequently they are of considerable 

 importance as rock-builders; some of the best known 

 genera are Labechia, Stromatopora, Stromatoporella, 

 Actinostroma, Clathrodictyon, Idiostroma, and Amphipora. 

 A few specimens, which are believed to be Stromatoporoids, 

 have been found in deposits of Mesozoic age. 



CLASS II. SCYPHOZOA 



The Scyphozoa (Scyphomedusse) or Acalephse include 

 the larger and more conspicuous jelly-fishes, such as 

 Aurelia, Rhizostoma, and Pelagia. They possess no hard 

 parts; nevertheless the impressions of some forms {e.g. 

 Rhizostomites) belonging to the Order Discomedusse have 

 been found in the Lithographic Limestone (Upper 

 Jurassic) of Solenhofen in Bavaria. 



Even in the oldest fossiliferous formations traces of 

 supposed Scyphozoa have been found; the most satisfactory 

 of these is the form from the Lower Cambrian of Sweden 

 referred to the Discomedusse, and named Medusina costata 

 ( = Medusites lindstroemi). Others, but of which the 

 nature appears to be somewhat doubtful, have been 

 described by Walcott from the Middle Cambrian of 

 Alabama. 



