20 BRITISH MARINE TESTACEOUS MOLLTJSCA : 



ACEPHALA PALLIOBKANCHIATA. 



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 laminse, on each side of the body of 

 the Lamellibranchiata ; they are the Brachiopoda of Lamarck, 

 and form two families in respect to the British iudigena, the 

 Terebratulidce, containing tliree genera and four species, and 

 the Craniadae, with oidy one genus and one species. 



This class of Acephala formed the great mass of the Mol- 

 lusca in the palseozoic 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; 110 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 



