MOLLUSCOIDA : DISTRIBUTION. 249 



confined to the sea. The marine Polyzoa are of almost uni- 

 versal occurrence in all seas. The fresh-water Polyzoa, how- 

 ever, not only differ materially from their marine brethren in 

 structure, but appear to have a much more limited range, 

 being, as far as is yet known, confined to the North Temperate 

 Zone. Britain can claim the great majority of the described 

 species of fresh-water Polyzoa, but this is probably due to the 

 more careful scrutiny to which this country has been subjected. 



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 ^athymetrical 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 existence in 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 Grapiolites 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 

 Paleozoic 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 Me- 

 sozoic 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 Bmchiopoda 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 De- 

 vonian 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 



