SUBDIVISIONS OF INVERTEBRATA. 315 



fifes, B acuities, Turrilites, &c. The only Palaeozoic genera are 

 Goniatites and Bactrites, of which the former is found ' from 

 the Upper Silurian to the Trias, whilst the latter is a Devo- 

 nian form. The genus Ceratites is characteristically Triassic, 

 but it is said to occur in the Devonian rocks. All the remain- 

 ing genera are exclusively Secondary, the genera Baculites, 

 Turrilites, Hamites, and Ptychoceras being confined to the Cre- 

 taceous period. 



Of the Dibranchiate Cephalopoda the record is less perfect, 

 as they have few structures which are capable of preservation. 

 They attain their maximum, as fossils, shortly after their first 

 appearance in the Secondary rocks, where they are represented 

 by the large and important family of the Belemnitida. Some 

 of the Teuthidce and Sepiadcz are found both in the Secondary 

 and in the Tertiary rocks, and two species of Argonaut have 

 been discovered in the later Tertiaries. No example of a 

 Dibranchiate Cephalopod is known from the Palaeozoic de- 

 posits, and the order attains its maximum at the present day. 



TABULAR VIEW OF THE CHIEF SUBDIVISIONS 

 OF THE INVERTEBRATA. 



SUB-KINGDOM I. PROTOZOA. 

 CLASS I. GREGARINID^E. 

 CLASS II. RHIZOPODA. 



Order i. Amcebea. 



2. Foraminifera. 



3. Radiolaria. 



4. Spongida. 

 CLASS III. INFUSORIA. 



Order i. Suctoria. 



2. Ciliata. 



3. Flagellata. 



SUB-KINGDOM II. CCELENTERATA. 

 CLASS I. HYDROZOA. 



Sub-class A. Hydroida. 

 Order i. Hydrida. 



2. Corynida. 



3. Sertularida. 



4. Campanularida. 



