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and proposes for it the name of Placodus laticeps, in reference to tnc 

 great breadth of the skull, which equals the entire length, each 

 measuring about 8 inches. All the teeth are implanted in distinct 

 sockets, according to the thecodont type of the Lacertian order. The 

 relation of the large temporal fossse and of the wide span of the 

 zygomatic arches, to the enormous muscular force required to work 

 the crushing machinery of the jaws, is pointed out. 



The structure of the bony nostrils, the orbits, the palate, with other 

 particulars of the cranial anatomy of the Placodus, is next described 

 in detail, and compared with the same characters in Nothosaurus, 

 Simosaurus, Pistosaurus, and other Muschelkalk reptiles. The 

 dentition of these Saurians, although, like Placodus, thecodont in 

 respect of implantation, is of the ordinary crocodilian type in 

 respect of form, adapted to the prehension of fishes ; and there are 

 no palatal teeth. But the author remarks that such teeth exist in 

 the triassic Labyrinthodonts, with a disproportionate magnitude of 

 certain teeth which offers a certain analogy with the dentition of 

 Placodus. An account of the microscopic structure of the dentine, 

 enamel, and osseous tissue of the Placodus is then given. 



The extreme and peculiar modification of the teeth, in respect to 

 form and size, adapting them to the crushing and pounding of hard 

 substances, and the association of the Placodus with conchiferous 

 mollusks in such abundance as to have suggested the terms * Muschel- 

 kalk,' ( Terebratuliten-kalk,' and the like, for the strata containing 

 them, concur in evincing the class whence the Placodi derived their 

 chief subsistence ; and the author points out the relation of a con- 

 stant disposition of the teeth, in all the known species, to the readier 

 cracking of shelly substance. A single row of teeth in the lower 

 jaw is always opposed to a double row in the upper one, playing, 

 with its strongest line of force, upon their interspace. Thus the 

 crushing force below presses upon a part between the two points of 

 resistance above, on the same principle on which a stick is broken 

 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 shell pressed between two opposite flat surfaces might 

 resist the strongest bite ; but, subjected to alternate points of press- 

 ure, its fracture is facilitated. 



