128 MANUAL OF THE MOLLUSCA. 
(Orthoceratide and Belemnitide) were respectively the first de- 
veloped. 
Amongst the Brachiopoda the hingeless genera attained their 
maximum in the palzozoic 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 their 
maximum while the Terebratulide were still few in number. 
The subdivision with calcareous spires disappeared with the 
Liassic period, whereas the genus Rhynchonella still exists. 
Lastly, the typical group, Terebratulide, 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 seyen times 
more numerous in the newer tertiary than in the oldest geo- 
logical system. The paleeozoic formations contain numerous 
genera of all the families with an open mantle; Cyprinide, 
Anatinide, 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 are represented in the paleeozoic strata by 
several genera closely allied to the diminutive Atlanta and Scissu- 
rella, and by others perhaps related to Janthina. The Naticidee 
and Calyptreide are plentiful, and there are several genera of 
elongated spiral shells referred to the Pyramidellide. In the 
secondary strata, holostomatous shells become plentiful; and in 
a few peculiar localities (especially Southern India) the genera 
of siphonated uniyalyes make their appearance in strata of 
Cretaceous age. Fresh-water Pulmonifera 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 Growps 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 hypothesis seems to have arisen out of an exclusive 
regard to the poverty of the Permian atid Triassic strata in England, where they 
separate, like a desert, the paleozoic from the “ neozoic”’ formations. The *‘ Permian” 
should never have been esteemed more than a division of the carboniferous 
system, and is poor in species, rather 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. 
