198 HISTORICAL PALAEONTOLOGY. 



Amongst the Molhtsca, the remains of Polyzoa may fairly be 

 said to be amongst the most abundant of all the fossils of the 

 Permian formation. The principal forms of these are the 

 fronds of the Lace-corals (Fenestella, Retepora, and Synodadia), 

 which are very abundant in the Magnesian Limestone of the 

 north of England, and belong to various highly characteristic 

 species (such as Fenestella retiformis, Retepora Ehrenbergi, and 

 Synodadia virgulacea). The Brachiopoda are also represented 

 in moderate numbers in the Permian. Along with species of 

 the persistent genera Discina, Crania, and Liygida, we still 

 meet with representatives of the old groups Spin/era, Athyris, 

 and Streptorhynchus ; and the Carboniferous Producta yet 

 survive under well-marked and characteristic types, though in 

 much-diminished numbers. The species of Brachiopods here 

 figured (fig. 135) are characteristic of the Magnesian Limestone 

 in Britain and of the corresponding strata on the Continent. 



Fig- 135 Brachiopods of the Permian formation, a, Producta horrida; b, Lingula, 

 Credneri; c, Terebratitla.elonga.ta.; rfand<% Camarophoria globulbia, (After King.) 



Upon the whole, the most characteristic Permian Brachiopods 

 belong to the genera Producta, Strophalosia, and 



The Bivalves (Lamellibranchiata) have a tolerably varied 

 development in the Permian rocks ; but nearly all the old 

 types, except some of those which occur in the Carboniferous, 

 have now disappeared. The principal Permian Bivalves 

 belong to the groups of the Pearl Oysters (Aviculidce) and the 

 Trigoniadce, represented by genera such as Bakewettia and 

 Schizodus ; the true Mussels (Mytilida), represented by species 

 which have been referred to Mytilus itself; and the Arks 

 (Arcada), represented by species of the genera Area (fig. 136) 

 and Byssoarca. The first and last of these three families have 

 a very ancient origin; but the family of the Trigoniadcz, though 



