266 THE LIAS AMMONITES. 



now living, and some of these have been carefully studied in relation to their structure and 

 morphology by Owen, Huxley, Hancock, and Davidson, in England, and by Vogt, Er. 

 Miiller, Lacaze-Duthiers, and Suess on the Continent, and by E. S. Morse in America. 



The Lamellibranchiata are well known to us as Oysters, Mussels, Cockles, and other 

 bivalve shells ; they have a two-lobed mantle, under whose folds four lamellae, or gills for 

 respiration, are concealed, and the circulation is carried on by a heart. The shell consists 

 of, with few exceptions, two valves, which correspond to the right and left sides of the 

 body, the dorsal region being placed under the hinge ; from the beak or umbo of each valve 

 of the shell numerous concentric lines of growth running parallel with the margin can be 

 traced; the inner differs from the outer surface by often exhibiting a mother-of-pearl 

 lustre, the cause of difference being seen on examining with a microscope transparent sec- 

 tions of the shell. The development of this class has been carefully studied by Loven, 

 Lacaze-Duthiers, and others, from whose observations it appears that important changes take 

 place in the evolution of the embryo, and during the subsequent growth of the Mollusk. 



The Ckphalophora, or head-bearing Mollusks, comprise the Gasteropoda, Pteropoda, 

 and Cephalopoda. 



The Gasteropoda have a body with a large muscular foot extended under the abdo- 

 men, adapted for creeping, and a head supporting tentacula and simple eyes. The 

 marine forms breathe by branchiae, and the lacustrine and terrestrial genera by a 

 pulmonary sac. The body is in general covered with a solid unilocular shell ; some are 

 naked, and others have an internal dorsal or branchial shield ; the morphology of these 

 animals has formed the subject of special studies for many observers, who have discovered 

 the important changes that take place in this class in the different genera that formed the 

 special objects of their investigations ; these belong, however, to a treatise on Embryology, 

 and are too numerous and complicated to be summarised here. The memoirs by Eol, 

 Bobretzky, Gegenbaur, Loven, Agassiz, and Lankester, abound with important details 

 regarding the evolution of Gasteropoda. 



The Pteropoda have the body organised for swimming, the mouth closed above, and 

 the branchiae external ; the shell when present is thin, unilocular, and generally iuopercu- 

 late ; these small, soft, floating Mollusks are entirely marine, and swim by the contractions 

 of two lateral musculocutaneous fins which support the branchiae. There appears to be 

 a very close agreement amongst Gastropods and Pteropods in the general character of 

 their larvae, and the changes they pass through in subsequent stages of growth. 



The Cephalopoda form the highest class, and their general structure has already been 

 described in the systematic part of this work, where it was shown that the only existing 

 representatives of the order Tetrabranchiata are the different varieties of Nautilus pompi- 

 lius found in southern seas. The family to which this genus belongs is one of the oldest, 

 as it can be traced through all the fossiliferous rocks down to those of Cambrian age. In 

 the Silurian rocks we find closely allied forms, which differ from Nautilus in the degree of 

 curvature their shell-cone assumes, as in Lituiles, Gyroceras, Trochoceras ; or straightness of 



