ANIMALS. 67 
SuB-K1INcGpoM MOLLUSCA, Cuvier. 
Of the six great groups or classes into which this section of the Animal Kingdom 
is divided, viz. Tunicata, Palliobranchiata, 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.’ 
Bracuropopa, Dumeril. 
PoLyMaRIA, Deshayes. 
Diagnosis—“ The Brachiopoda are 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 7unicata, 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 Verebratula 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 cesophageal 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 
! Dict. des Se. Nat., t. xxxii, p. 298, 1824. 
* Lectures on the Comparative Anatomy and Physiology of the Invertebrate Animals, p. 269. 
’ Transactions of the Zoological Society, vol. i, part i; and Lectures on the Comparative Anatomy and 
Physiology of the Invertebrate Animals, p. 280. 
4 Memoirs of the Geological Suryey, vol. ii, part i, p. 369. 
