ANIMALS. 67 



Sub-kingdom MOLLUSCA, Cuvier. 



Of the six great groups or classes into which this section of the Animal Kingdom 

 is divided, viz. Tunicata, PaUiobranchiata, Lamellibranchiata (ordinary bivalves), 

 Gasteropoda (univalves), Cephalopoda, and Pteropoda, only two, the first and the last, 

 are unknown as having existed during the Permian epoch. 



Class Palliobranchiata, Blainville.^ 



Brachiopoda, Dumeril. 

 PoLYMARiA, Deshayes. 



Diagnosis. — " The Brachiopoda arc defended by a bivalve shell, have two long 

 spiral arms developed from the sides of the mouth, and respire by means of their 

 vascular integument or mantle." (Owen.)^ 



With the exception of the Tunicata, the Palliobranchs or Brachiopods are generally 

 considered as constituting the lowest class of molluscous animals. They have existed 

 during all the organic periods of our planet ; but have evidently been more numerous 

 and more diversified in form in remote ages than at present. Only a few kinds are 

 now living in the British seas. 



Before describing the various genera and species of Palliobranchiate shells, to be 

 noticed in this Monograph, it is proposed to enter into some particulars on the principal 

 structures of the- class in general. 



Professor Owen having shown that, of the two valves with which a Brachiopod is 

 provided, the large or perforated one, as in Terehratula caput- serpentis, stands in a 

 direct relation to the ventral parts of the animal, that is, the principal nerves are given 

 off from that side of the oesophageal collar nearest to it, and the mouth is turned 

 towards it, while the hearts are situated nearest to the opposite one ;3 it necessarily 

 follows, that the perforate valve should be distinguished by the name ventral, and the 

 opposite or imperforate one by the name dorsal. Mr. Salter* has already used 

 these terms in this sense; and they will be employed as such in the following 

 pages, though not generally, as other terms can, in many cases, be more conveniently 

 adopted. 



The articulation of the valves is effected by a pair of teeth or condyles attached to 



1 Diet, des Sc. Nat., t. xxxii, p. 298, 1824. 



^ Lectures on tlie Comparative Anatomy and Physiology of the Invertebrate Animals, p. 2G9. 



2 Transactions of the Zoological Society, vol. i, part i ; and Lectures on the Comparative Anatomy and 

 Physiology of the Invertebrate Animals, p. 280. 



* Memoirs of the Geological Survey, vol. ii, part i, p. 369. 



