20 BRITISH MARINE TESTACEOUS MOLLUSCA: 
ACEPHALA PALLIOBRANCHIATA. 
A few general remarks on the Palliobranchiate Mollusca, 
in addition to those in the synopsis of my classification, will 
be a useful reference in respect of this class of acephalous 
animals. I include in the general matter on the Pallio- 
branchiate Mollusca, the meagre notices and references to the 
very few recent species of this tribe that now exist; these, 
however, contain all that is known of the organization of this 
numerous group of one of the epochs which preceded that in 
which our orb has been described as “rudis indigestaque 
moles.” For the reasons stated in the analysis of the Mol- 
lusca, I have reversed the position of this section of the 
bivalves, and made it the alpha instead of the omega of the 
Acephala. 
These animals derive their appellation from the respiratory 
apparatus forming a vascular network on the substance of 
the mantle, instead of being provided with the distinct sym- 
metrical, usually double lamjnz, on each side of the body of 
the Lamellibranchiata ; they are the Brachiopoda of Lamarck, 
and form two families in respect to the British indigena,—the 
Terebratulide, contaiming three genera and four species, and 
the Craniade, with only one genus and one species. 
This class of Acephala formed the great mass of the Mol- 
lusca in the palzeozoic ages, as is proved by the almost infinite 
variety of their fossils, which are more numerous than all the 
other groups of Mollusca united. But in these latter days it 
would appear, from the excessive rarity of living species, that 
the race has nearly become extinct, or live in unapproachable 
depths ; I believe that not half a dozen species have ever been 
examined, and we can scarcely hope that several anomalies in 
the structure of these animals and their shells will ever be 
explained; no comparison can be made—the same animal 
must be observed. The British collector cannot hope to pos- 
sess more than five recent species, two of which, the Hypothyris 
psittacea and Terebratula cranium, may be considered almost 
