136 THE MIDDLE PAST. 



patella, ceritliium, and the like, being the more common 

 forms in the marine strata ; while planorbis, paludina, and 

 their congeners occur in the fresh-water limestones of the 

 Weald thick as their modern species do on the marls of our 

 lakes and marshes. The cephalopods now attain their 

 meridian, and in variety of form, size, and numbers, stamp 

 the period with one of its most peculiar aspects. Shell- 

 clad genera, like nautilus and ammonite, leave their cham- 

 bered habitations in myriads ; and naked genera, like the 

 cuttle-fishes, are evidenced by thousands of those internal 

 organisms (belemnites) which survive the decay of the softer 

 structures. It is indeed the "reign of ammonites" these 

 beautiful shells occurring in hundreds of specific forms, in 

 every stage of growth, and in the most diversified styles of 

 external ornamentation. Along the exposed shore, in the 

 land-locked bay, and out in the open waters of the old 

 oolitic seas, these predaceous shell-clad cephalopods reign 

 the lords of niolluscan life, and mark the culmination of an 

 order which now finds its only representative in the plain- 

 looking nautilus of the Southern Ocean. 



The fishes to which we next ascend belong exclusively 

 to the great placoid and ganoid divisions the soft-scaled 

 orders (the ctenoids and cycloids) of the newer epoch being 

 as yet unrepresented in oolitic waters. The placoids are 

 chiefly rays and sharks, whose teeth (liybodus, acrodus, 

 ganodm, &c.) and fin-spines (aster acanthus, nemacanthus, 

 and myriacanthus) were the only preservable portions of 

 their uncalcified skeletons. Many of these, like the cestra- 

 cion of the Australian seas, were evidently fitted for the 

 crushing of crustaceous and testaceous animals, others for 

 the prehension of fishes, while some, more slenderly armed, 

 gorged themselves, like their modern congeners, on the 

 squids and cuttle-fishes that then thronged the ocean. The 

 ganoids, now more ichthyic in their aspect, appear in nu- 



