Till'. HEAD OB V M'.w FOSSIL WIMVL. 17". 



It differs from the Pkaiosaurus in the form and relative size 

 of the intermaxillary and upper maxillary bones — in the 

 form of the lower jaw — form of the teeth — mode in which 

 they close — and manner in 'which the lower maxillary nerve 

 is transmitted to the teeth, which, in the Plesiosaurus, is by 

 foramina dispersed irregularly along the outer edge of tin- 

 lower jaw. 



It appears most nearly to approach the Saurocephaku, a 

 genus founded on a single dental bone, discovered in a ca- 

 mera on the river .Missouri, hear Soldiers 1 river, hy Sergeant 

 Gass, who accompanied Lewis and Clark in their expedi- 

 tion, and presented by the latter gentleman to the Society, 

 and now in their cabinet*. It resembles this animal in the 

 teeth closing like incisors — in the foramina for the transmis- 

 sion of nerves and blood-vessels to the teeth of the lower 

 jaw being in a regular series on the inner side of the dental 

 bone near the alveolar edge — and in the young teeth enter- 

 ing the old directly in the centre and not at the side, as in 

 the other animals of this order, in the Crocodile. &c. 



It differs however from the Saurocephalw in the teeth 

 being in distinct alveoli, while the teeth in the latter are 

 described as being -fixed in a longitudinal groove" -in close 

 contact throughout," -there being no distinct alveoli." 

 Our animal differs also in the groove on (lie inside of the 

 denial bone for the accommodation of the inferior maxillary 



nerve (and which is made a generic character) being ab- 

 sent, in the form of the teeth, and no doubt in many other 

 particulars, which a want of opportunity for comparison and 

 moie perfect specimens makes j( impossible lor us to point 

 out 



Under these circumstances we venture to propose for it 

 a new genus, under the name of S\i hod on, and will dedi- 



* We regret much not having an opportaoil mining this specimen : »•• 



are compelled to depend upon Ihi account of il by Richard Harlan, M.l>. in the 

 Journal of the Academy of Natural Science! "i Philadelphia) Vol. m. Part ll 

 von. in. — G r. 



