418 MANUAL OF THE MOLLUSOA. 



Amongst the Brachiopoda the hinge-less genera attained their maximum 

 in the palaeozoic age, and only three now survive, {Lingula, Discina, Crania,) 

 — the representatives of as many distinct families. Of the genera with arti- 

 culated valves, those provided with spiral arms appeared first and attained 

 their maximum while the Terebratulidce were still few in number. The sub- 

 division with calcarious spires disappeared with the Liassic period, whereas 

 the genus Rhyuclionella still exists. Lastly, the typical group, Terebratu- 

 lidce, attained its maximum in the chalk period, and is scarcely yet on the 

 decline. The ntimber of sub-genera (as well as genera,) in each system, is 

 stated in the preceding table, because this group shews a tendency to 

 " polarity," or excessive development at the ends of the series." 



The genera of o-dinary oivalves {Conchifera) are seven times more nu- 

 merous in the newer tertiary than in the oldest geological system. The 

 palaeozoic formations contain numerous genera of all the families with an 

 open mantle ; Cyprinidce, Anatinidce^ and the anomalous genus Conocardium. 

 The mass of siphonated bivalves do not appear till the middle of the second- 

 ary age, and are only now at their maximum. 



The Gasteropoda are represented in the palajozoio strata by several 

 genera closely allied to the diminutive Atlanta and Scissurella, and by others 

 perhaps related to lanthina. The Nuticidce, and Galyiitrteidce are plentiful, 

 and there are several genera of elongated spii'al shells referred to the Pyra- 

 midsllidce. In the secondary strata, holostomatous shells become plentiful ; 

 and in a few peculiar localities (especially Southern India) the genera of 

 siphonated univalves make their appearance in strata of Cretaceous age. 

 Fresh-water Pulmonifera of the recent genus Pliysa occur in the Purbeck j 

 strata, but the marine air-breathers and land-snails have not certainly been j 

 found in strata older than the Eocene tertiary. ! 



Order of Succession of Groups of Shells. — It has been already pointed 

 out that animals which are closely allied in structure and habits, rarely live \ 

 together, but occupy distisct areas, and are termed "representative species." 

 The same thing has been observed in the distribution of fossils ; the species \ 

 of successive strata are mostly representative. 



At wider intervals of time and space, the representation is only generic, | 

 and the relative proportions of the larger groups are also changed. j 



1*116 succession of forms is often so regular as to mislead a superficial j! 

 observer; whilst it affords, if properly investigated, a Valuafble clue to the 

 afiiaities of problematic fossils. 



* See the anniversary address of Prof. E. Forbes to the Geological Society of Lon- 

 don, Feb. 1854, p. 63. 'Jhe hypothesis seems to have arisen out of an exclusive regard; 

 to the poverty of the Permian and Triassic strata in England, where they separate, 

 like a deseirt, the palaeozoic from the " neozoic ' formations. The " Permian " shouldj 

 never have been esteemed more than a division of the Carboniferous system, and 

 l>oor ill species, rather tJian in types. 1 he Trias must be studied in Germany, or ii 

 the collection of Dr. Klips tein [\n the Brit. Museum) to be properly aiipreclated. 



