CEPHALOPODA. 113 



vegetarians ; while in marine shells, the mouth is interrupted by a notch 

 or prolonged into a canal, and the animal is carnivorous. 



Molluscs are separable into the Cephalous and Acephalous. The 

 former are nearly all univalves, and are subdivided, according to the 

 modifications of the locomotive organs, into Cephalopoda, Gasteropoda 

 and Pteropoda. Headless Molluscs are bivalves chiefly , and are divided into 

 Lamellibranchia (or Conchifera), Brachiopoda, Tunicata, and Bryozoa. 



The natural families of Molluscs now verging toward extinction, are 

 the Rhynchonellidse., Trigoniadx, and Nautilidse,. The following have al- 

 ready passed away : Productidse, Orthidse, Spiriferidse, Hippuritidee, 

 Orthoceratidse, Ammonitidse, and Belemnitidse. Lamellibranchs have 

 superseded Brachiopods ; and Gastropods vastly outnumber Cephalopoda. 

 The extinct families and genera attained their maximum more rapidly 

 than their minimum. 



The remains of shell-bearing Molluscs are the most common of all 

 fossils, and afford the most complete series of " medals" for the identifi- 

 cation of strata. The sub-kingdom started an unfolded type. All its 

 grand divisions, even to the highest, are represented in the lowest rocks. 

 Both the highest and lowest groups were most abundant in the Palseo- 

 zoic age ; the ordinary bivalves and univalves attain their climax in exist- 

 ing seas. Fossil shells number about three-fourths of living species— 15,000 : 

 of these 500 are land, 800 fresh-water, and over 13,000 marine. They 

 passed their culmination in the latter half of the Mesozoic age; but the 

 largest number of species occur in the Miocene. The average duration 

 of the marine species is one-third the length of a geological period, 

 which accounts for the fact that so few have a world-wide distribution. 

 The life of land and fresh-water shells is of longer average extent. Tertiary 

 fresh-water shells are Old World forms; while Tertiary land-shells are 

 American in character. 



CLASS I.— CEPHALOPODA. 



The Cephalopods are the most highly organized of Molluscs. We 

 recognize elements which belong to the highest and lowest forms of 

 life : the beak and complex eye, the tongue and ear, the crop and gizzard, 

 the partial segmentation of the- yolk, the Chameleon habit of changing 

 color, the cartilages — an analogue of the spine — point to the Vertebra- 

 tes ; while the first draught of the pharyngeal ganglion is seen in the 

 Rotifera, the sucker-bearing arms have their first outline in the Hydra, 

 the phragmocone represents the calcareous axis of a coral, and if the 

 mouth of the Cuttle-fish with its arms be separated from the head, we 

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