SUBDIVISIONS OF INVERTEBRATA. 361 



and Tertiary periods to the present day. The Orthoceratidce 

 died out much sooner, being exclusively Palaeozoic, with the 

 exception of the genera Orthoceras itself and Cyrtoceras, which 

 survived into the commencement of the Secondary period, 

 finally dying out in the Trias. 



The second family of the Tetrabranchiata viz., the Ammo- 

 nitidce is almost exclusively Secondary, being very largely 

 represented by numerous species of the genera Ammonites, 

 Ceratites, 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, Hamties, and Ptychoceras being confined to the 

 Cretaceous 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 Belemnitidce. Some 

 of the Teuthida 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. Monera. 



2. Amcebea. 



3. Foraminifera. 



4. Radiolaria. 



5. Spongida. 

 CLASS III. INFUSORIA. 



Order i. Suctoria. 



2. Ciliata. 



3. Flagellata. 



