40 CCELENTERATA. HYDROZOA. 



group. The Hydrocorallinse in which the skeleton is 

 calcareous, are also placed here, Millepora being the best 

 known form, it ranges from the Eocene to the present day. 



The Scyphomedusse includes such jelly-fishes as 

 Aurelia, Rhizostoma, and Pelagia, but as they possess no 

 hard parts it is not to be expected that they should be 

 found fossil ; nevertheless the impressions of some forms 

 (e.g. Rhizostomites) have been found in the Lithographic 

 Slate of Solenhofen. 



The Siphonophora includes Physalia ('Portuguese 

 Man-of-War'), Physophora, and other forms, and is 

 unknown in the fossil state. 



SUB-CLASS. GRAPTOLITOEDEA. 



The Graptolitoidea includes the graptolites ; these are 

 altogether extinct, being found only in the Lower Palaeozoic 

 rocks, where owing to their abundance and to the limited 

 range in time of both genera and species, they are of 

 immense importance to the stratigraphical geologist. 

 They occur most commonly in argillaceous rocks, especially 

 in black shales, whilst they are rare in sandstones and 

 limestones. The graptolites were, like the modern 

 Sertularia, compound animals, consisting of a number of 

 polypites united' by a common ccenosarc, the whole being 

 protected by a skeleton of chitin. But the original 

 material of the skeleton is never preserved in the fossil 

 state, its place being taken by carbon or iron pyrites. 

 The entire skeleton is termed the polypary; this in a 

 simple form like Monograptus, consists of the following 

 parts. A more or less cylindrical tube known as the 

 common canal (fig. 8 b, c), extends the whole length of 

 the animal, the wall being termed the periderm. From 



