GraptoUtes, — Spongida — Sponges. \ \ 3 



3. The Geaptolitoidea form the group of the Hyclrozoa of most GALLERY 

 Talue to the geologist, for in working out the problems of zonal ^ 

 stratigraphy, the graptolites in the Lower Paleozoic rocks play ^^"-case 

 the same part as do the Ammonites and Brachiopods in the cases 

 Mesozoic. In Table-case 10 are shown all the leading forms ot ® * 1®- 

 Graptolites, also a valuable series of type-specimens. 



The first important Graptolite fauna in the British series comes 

 from the Skiddaw Slates, one of the lowest members of the 

 Ordovician system, where we find the leaf-like Phijllograptus, the 

 bifid Didymograptm, the radiating Loganograptus, and others, such 

 as Biplograptus, with double series of " hydrothecse " and the 

 cup- shaped expansions protecting the polypes. Dictyograptus 

 {Dictyonema) socialis, from the Tremadoc beds, is older, and shows 

 that graptolites lived in the Cambrian. 



In the succeeding Llandeilo and Bala series most of the same 

 genera are present, with simple '' monoprionidian" forms, such as 

 .BicranograptuH, in which there is only a single row of hydrothecae. 



In the Silurian system graptolites with single series became the 

 prevalent forms, notably the genus Graptolithus and Rastrites, 

 in the latter of which the hydrothecae are not closely packed along 

 the stem. A good series of these genera from the shales of 

 Dumfriesshire is exhibited, and among them are some slabs of the 

 ^^ Daivsonia,''^ which are probably the reproductive polypites, or 

 gonophores, of graptolites. 



4. The Thecaphoea are perhaps represented by Bendrograptus, 

 Callograptus, etc. 



Y.— PORIFERA. 



SPONGIDA— SPONGES. 

 Sponges are aquatic organisms of very varied form and size, Fossil 

 inhabiting both fresh and salt water. The sponge-animal or Sponges. 

 sponge-flesh, as shown in the recent, living forms, is a soft ® ^_ 

 gelatinous substance, consisting of microscopic cells, of various ^^^^^ 

 forms and functions, which are arranged so as to form an outer cases 

 membrane, the "ectoderm," of a single layer of flattened cells ; ^^^^^^^^^ 

 an inner membrane, or "endoderm," also of a single layer; and 7 4 8. 

 an intermediate layer of varying thickness, usually known as the 

 '* mesoderm." The sponge-body is traversed by canals which open 

 at the surface, and are lined by epithelial cells, some of which are 



I 



