124 MANUAL OF ZOOLOGY. 



The Sertularida and Campanularida are not certainly known 

 to occur in a fossil condition. The fossils called Dendrograms us, 

 Callograpsus, Ptilograpsus, and Dictyonema, all at present placed 

 amongst the Graptolites, are, however, not improbably truly 

 referable to the Sertularida. 



There can be little doubt but that the large and singular 

 family of the Graptolitidce, should really be looked upon as 

 extinct Hydrozoa, though good authorities still place them 

 amongst the Polyzoa. As regards their distribution two facts 

 are chiefly noticeable. In the first place, no Graptolite, except 

 the doubtful genus Dictyonema, has hitherto been found to occur 



above the Silurian rocks. 

 * lie Graptolites may therefore 

 be regarded as characteristic 

 fossils of the Silurian period, 

 though they commence their 



existence in the Upper Cam- 

 Fig. &.--Didy*no g rap S us V-fractus. ^-^ Rockg> Secondlyj the 



diprionidian Graptolites, or those with a row of cellules on each 

 side (genera Diplograpsus, Climacograpsus, and Dicranograpsus], 

 have in Bohemia alone been certainly shown to occur above 

 the horizon of the Lower Silurian rocks. The common genus 

 Didymograpsus (comprising the " twin " Graptolites, fig. 38) 

 is still more characteristic of the Lower Silurian period. In 

 Didymograpsus the polypary consists of two lateral symmetrical 

 branches, with cellules on one side only, springing from a 

 central point or base, which is usually marked by a little spine 

 or " radical." 



CHAPTER XIII. 

 ACTINOZOA. 



i. GENERAL CHARACTERS OF THE ACTINOZOA. 2. CHAR- 

 ACTERS OF THE ZOANTHARIA. 3. ZoANTHARIA MALACO- 

 DERMATA. 4. ZoANTHARIA SCLEROBASICA. 5. ZOAN- 

 THARIA SCLERODERMATA. 



CLASS II. ACTINOZOA. The Actinozoa are defined an Ccelen- 

 terata with a differentiated digestive sac opening below into the 

 somatic cavity, but separated from the body-walls by an interven- 



