288 Royal Society : — 



PROCEEDINGS OF LEARNED SOCIETIES. 



ROYAL SOCIETY. 



March 11, 1858. — Dr. Hooker, Vice-President, in the Chair. 



" Description of the Skull and Teeth of the Placodus laticeps, Ow., 

 with indications of other new species of Placodus, and evidence of the 

 Saurian nature of that extinct genus." By Prof. Richard Owen, 

 F.R.S. &c. 



The author premises a brief sketch of the history of the discovery of 

 the fossils referred by Count Minister, and Professors Agassiz, Bronn, 

 and Meyer to the Pycnodont family of Ganoid Fishes, under the 

 generic name of Placodus; and then enters upon the anatomical 

 grounds on which he concludes that the Placodus is a Saurian reptile. 

 These are stated to be, principally, — 1, distinct external bony nostrils, 

 divided by an ascending process of the premaxillary, and bounded by 

 that bone, the maxillaries and nasals ; 2, orbits circumscribed below 

 by the superior maxillary and malar bones ; 3, temporal fossae of 

 great size and width, bounded externally by two zygomatic arches, 

 the upper formed by the postfrontal and mastoid, the lower formed 

 by the malar and squamosal ; 4, the tympanic bone formed by one 

 bony piece, with a trochlear lower articular surface ; the limitation of 

 the teeth to the premaxillary, maxillary, palatine, and pterygoid 

 bones, in the upper jaw, with a demonstrated absence of a median 

 vomerine series, such as exists in the true Pycnodonts. With these 

 proofs of the reptilian nature of the Placodus, Prof. Owen combines 

 others exemplifying its affinities to the Lacertian order, and more 

 especially with that modification of Lacertia exemplified by the 

 extinct genus Simosaurus, from the Muschelkalk. 



The author then describes the dentition of the upper jaw of the 

 specimen of Placodus, demonstrating the foregoing characters. It 

 includes two premaxillary and three maxillary teeth, forming an 

 outer or marginal series, and two teeth of larger size, forming an inner 

 or palatal series, the last of which is described as the largest grinding 

 tooth in proportion to the size of the head, hitherto known in the 

 animal kingdom. 



From the cranial and dental characters the author deduces the 

 specific distinction of his specimen from previously described Placodi, 

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

 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 fossae 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 





