128 MANUAL OF THE MOLLUSCA. 



{Orthocerafidce and Belemnitidce) were respectively the first de- 

 veloped. 



Amongst the BracMopoda the hingeless 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 articulated valves, those 

 provided with spiral arms appeared first and attained theii 

 maximum while the Terehratulidce were still few in number. 

 The subdivision with calcareous spires disappeared with the 

 Liassic period, whereas the genus Ehynchonella still exists. 

 Lastly, the typical group, Terehratulidce, attained its maximum 

 in the chalk period, and is scarcely yet on the decline. The 

 number of sub -genera (as well as genera) in each system is 

 stated in the preceding table, because this group shows a ten- 

 dency to ''polarity," or excessive development at the ends of 

 the series.* 



The genera of ordinary bivalves {Conchifera) are seven times 

 more numerous in the newer tertiary than in the oldest geo- 

 logical system. The jDalseozoic formations contain numerous 

 genera of all the families with an open mantle ; Cyjorinidce, 

 Anatinidce, and the anomalous genus Conocardium. The mass 

 of siphonated bivalves do not appear till the middle of the 

 secondary age, and are only now at their maximum. 



The Gasteropoda a.re represented in the palaeozoic strata by 

 several genera closely allied to the dim i n utive A tlanta and Scissu- 

 rella, and by others perhaps related to lanthina. The NatiddcB 

 and Calyptroiidce are plentiful, and there are several genera of 

 elongated spiral shells referred to the Fyramidellidce. 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 Fulmonifera of the recent genus 

 Physa occur in the Purbeck strata, but the marine air-breathers 

 and land- snails have not certainly been 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 



* See the anniversary address of Professor E. Forbes to the Geological Society of 

 London, Feb., 1854, p. 63. The hyi^othesis seems to have arisen out of an exclusive 

 regard to the poverty of the Permian and Triassic strata in England, where tliey 

 separate, like a desert, the palaeozoic from the " neozoic " formations. The " Permian" 

 should never have been esteemed more than a division of the carboniferous 

 system, and is poor in species, ratlier than in types. The Trias must be studied in 

 Germany, or in the collection of Dr. Klipstein (in the British Museum) to be properly 

 appreciated. 



