a new Fossil Animal. 
581 
only preliminary. In this, as was strikingly exemplified in the 
history of the discoveries connected with the former genus, we 
doubt not but that an early and unreserved communication of 
materials, which from the circumstances of the case cannot but be 
imperfect, will soon lead to the removal of the present deficiencies, 
and for the reasons stated in the beginning of this memoir, we 
trust that what has already been ascertained, which indeed com- 
prises the most material points connected with the trunk and ante- 
rior extremities, will not be found entirely devoid of interest. 
PLESIOSAURUS. 
General character . A marine animal, intermediate in its struc- 
ture between the Ichthyosaurus and Crocodile. 
Teeth. Not having as yet obtained a head of this animal, we 
cannot speak with certainty on this subject ; but a peculiar tooth, 
not belonging to any species of Ichthyosaurus, yet evidently of the 
Crocodilian type, occurs in the lias, and may with great probability 
be referred to this animal. 
Jaw and Head bones . Not as yet discovered.* 
Vertebrae. These agree more nearly with the Crocodile than with 
the Ichthyosaurus ; we shall therefore compare them with the 
former type; stating first their points of agreement, and then of 
difference. 
They agree with those of the Crocodile in having their annular 
part attached to the body by sutures ; those in the younger speci- 
mens allow the two parts to separate completely, and form a clean 
socket not unlike that in the Ichthyosaurus, but in the older speci- 
mens they are completely anchylosed : they farther agree in having 
* Since this paper went to press a few very imperfect specimens have occurred, which 
exhibit very little of an illustrative character, excepting that in this animal the teeth 
were carried in distinct alveoli, and not in a continuous sulcus, as in the Ichthyosaurus. 
