462 BRITISH FOSSIL REPTILES. 



All the ribs or pleurapophyses, where present or preserved, are monocipital. Iii 

 Leiodo7i the basisphenoid is concave, almost canaliculate alons: the middle of the 

 under surface, devoid of any ophidian median ridge; the pair of hypapophyses 

 abut against the pair from the hasioccipital, but leave the broad truncate ends of 

 these free, as in Amblyrliynchus. 



In Leiodon, as in Mosasauriis, there is a large 'foramen pineale' which, as in 

 Mo7iifor, is wholly in the parietal. This bone bifurcates posteriorly ; its prongs 

 extend backward, outward, and articulate with the mastoid, which curves outward 

 and downward to join the squamosal, and, with it, forms the articular surface for 

 the tympanic. Anteriorly the squamosal unites with the postfrontai, 12. The long 

 and wide temporal fossje are bounded, externally, as in Lacertians, by a long and 

 narrow zygomatic bridge, in the composition and proportions of which Leiodon 

 most rescml)les the Monitors and Iguanas. 



Leiodon, like other Mosasauroids, has two pairs of limbs, of the natatory type ; 

 the tegumentary sheath is supported by five digits, in both fore and hind fins. The 

 phalangeal formula is, in the main, Lacertian (fig. 1). 



In the single occipital condyle and the composite structure of the mandible the 

 Mosasaurians are Licptilian ; in the proccelian vertebrse they accord with the existing 

 representatives of the class; in the double occipital hypapophyses, in the bifurcate 

 and perforate parietal, in the columella, in the composite formation of the suspensory 

 joint of tlie tympanic, in the type of the tympanic, in the frame of the parial nostrils, 

 in the composition of the mandible, and in the structure and attachment of the 

 teeth they are Lacertian; in one special dental modification they are lyuanian ; in 

 another they are 3Ioniforial. In the broad cemental basis of the enamelled tooth? 

 in the more extensive fixation of the pterygoids and ossification of the roof of the 

 mouth, in the large proportion of the vertebral column devoid of zygapophyses, in 

 the confluence of the haemal arch with the centrum of certain of the caudal vertebrae, 

 in the natatory character of the fore and hind limbs, they are Mosasaurian. But 

 they do not seem to me to be entitled, through the last category of modifications, 

 to the rank of an order in the reptilian class. 



The order Lacertilia, in the class Reptilia, is a taxonomic equivalent of the order 

 Carnivora or Ferse in the mammalian class. 



In the Ferae there is a group which, by modifications of the skull, teeth, 

 vertebrae, and, especially, limbs, takes rank as a suborder or subordinate group, 

 viz. the Pinnipedia or Phocida?. I estimate the Mosasaurians in the Lacertian 

 on er to be equivalent to the Seals in the Ferine order. 



