2/6 MANUAL OF ZOOLOGY. 



The Tunicata are cosmopolitan in their distribution, and are 

 found in all seas, the Mediterranean appearing to be especially 

 rich in members of this class. Four genera are pelagic in their 

 habits, and several are found in the Arctic regions. 



The Brachiopoda, though of very partial occurrence, have a 

 wide range in space, being found both in tropical seas and in 

 the Arctic Ocean. Their bathymetrical range is also very wide, 

 extending from the littoral zone, almost to the greatest depths 

 at which animal life has hitherto been detected. 



DISTRIBUTION OF MOLLUSCOIDA IN TIME. The Polyzoa 

 have left abundant traces of their past existenceln the stratified 

 series, commencing in the Lower Silurian Rocks and extending 

 up to the present day. The Oldhamia of the Cambrian Rocks 

 of Ireland, and the Graptolites have been supposed to belong 

 to the Polyzoa, but the former is very possibly a plant, and the 

 latter should be referred to the Hydrozoa. Of undoubted Po- 

 lyzoa, the marine orders of the Cheilostomata and Cyclostomata 

 are alone known with certainty to be represented. Several 

 Palaeozoic genera such as Fenestella (the Lace-coral), Ptilo- 

 dictya, Ptilopora, &c. are exclusively confined to this epoch, 

 and do not extend into the Secondary Rocks. Amongst the 

 Mesozoic formations, the Chalk is especially rich in Polyzoa, 

 over two hundred species having been already described from 

 this horizon alone. In the Tertiary period, the Coralline Crag 

 (Pleiocene) is equally conspicuous for the great number of the 

 members of this class. 



The Tunicata, from the nature of their bodies, are not known 

 to occur in a fossil condition. 



The Brachiopoda are found from the Cambrian Rocks up to 

 the present day, and present us with an example of a group 

 which appears to be slowly dying out. Nearly two thousand 

 extinct species have been described, and the class appears to 

 have attained its maximum in the Silurian epoch, which is, 

 for this reason, sometimes called the " Age of Brachiopods." 

 Numerous genera and species are found also in both the 

 Devonian and Carboniferous formations. In the Secondary 

 Rocks Brachiopoda are still abundant, though less so than in 

 the Palaeozoic period. In the Tertiary epoch a still further 

 diminution takes place, and at the present day we are not 

 acquainted with a hundred living forms. Of the families of 

 Brachiopoda, the Productidce, Strophomenida, and Spiriferidcz 

 are the more important extinct types. Of the genera, the most 

 persistent is the genus Lingida, which commences in the Cam- 

 brian Rocks, and has maintained its place up to the present 

 day, though it appears to be gradually dying out. 



