298 MANUAL OF ZOOLOGY. 



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. — lYi^ 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 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 Poly- 

 zoa, the marine orders of the Cheilostomata and Cydostomata 

 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 Productidcs, Sirophomenidce, and Spiriferidce 

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

 persistent is the genus Lingula, 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. 



According to Woodward : — "The hingeless genera attained 

 their maximum in the Palaeozoic age, and only three now sur- 

 vive {Lingula, Discina, Crania) — the representatives of as many 

 distinct families. Of the genera with articulated valves, those 



