220 PALEONTOLOGY 



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 molluscivorous 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 might resist the strongest bite, but subjected to alternate 

 points of pressure its fracture would be facilitated * 



Genus Tanystkopileus. 



Sp. Tanystrophmus conspicuus, H. von Meyer. — Certain long, 

 slender, hollow bones (fig. 70, a), from the German muschel- 

 kalk, were referred by Count Munster to the class Rcptilia, 

 under the name of Macroscdosaurus, under the impression that 

 they were bones of the limbs. H. von Meyer subsequently. 

 in more perfect specimens, observing that each slightly ex- 

 panded extremity of the long bone was terminated by a sym- 

 metrical oval concave articular surface, surmounted by a pair 

 of symmetrical lateral incurved plates, resembling confluent 



* Previous to the writer's Memoir on Placodus i» the Philosophical Trans- 

 actions (1858), all palaeontologists hail referred the genua to the pycnodonl 

 order of fish^ 



