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240 PALEONTOLOGY. 



upon detached vertebrae. The twelve or more evidences of 

 Placodus, afforded by bone as well as tooth, are all portions of 

 the skull. It is possible that some of the singularly modified 

 vertebrae from the muschelkalk, next to be described, may 

 belong to the Placodus ; and the same surmise suggests itself 

 in reference to some of the limb-bones from the muschelkalk 

 that cannot be assigned to other known saurian genera. 



The obvious adaptation of the dentition of Placodus to the 

 crushing of very hard kinds of food, its close analogy to the 

 dentition of certain fishes known to subsist by breaking the 

 shells of whelks and other shell-clad Mollusks, and the cha- 

 racteristic abundance of fossil shells in the strata to which the 

 remains of Placodus are peculiar, concur in producing the 

 belief that the species of this genus were reptiles frequenting 

 the sea-shore, and probably good swimmers. But as at present 

 we have got no further than the head and teeth in the recon- 

 struction of this mezozoic form of moluscivorous reptile, the 

 present notice will conclude with a remark suggested by the 

 disposition and form of the teeth. In all the species, under 

 the rather wide range of specific varieties of the dentition, 

 there are two rows of the crushing teeth in the upper jaw, and 

 only one row in the lower jaw, on each side of the mouth ; 

 and the lower row plays upon both upper rows, with its 

 strongest (middle) line of force directed against their inter- 

 space. Thus the crushing force below presses upon a part 

 between the two planes or points of resistance above, on the 

 same principle on which we break a stick across the knee ; 

 only here the fulcrum is at the intermediate point, the moving 

 powers at the two parts grasped by the hands. It is obvious 

 that a portion of shell pressed between two opposite flat sur- 

 faces would resist a stronger bite than if subjected to alternate 

 points of pressure. 



Genus Tanystkopheus. 



Sp. Tanystropliwus conspicuus,H.\ o\\ Meyer. — Certain long, 



