a new Fossil Animal . 
575 
against the cavity left by the prolonged lip of the socket into which 
the annular part of the vertebras is set. 
The twenty succeeding vertebras have two articulating tubercles 
to support the ribs, and must be assigned to the middle and 
posterior dorsal and lumbar. The upper of these tubercles is at 
first placed close to the lip of the annular socket, and thence assumes 
gradually a lower and lower position on the side, so as to approach 
more nearly to the inferior ; at length near the fortieth the two 
tubercles run together and form a single ridge. 
The 18th, 25th, and 38th, have been selected for representation, 
as exhibiting the mean form of these vertebras and their passage 
into the adjoining forms on either side. 
The changes of position in these points of articulation at once 
affected the sweep of the ribs in forming the cavity of the thorax 
and abdomen, and their motion in opening it in respiration. 
The vertebras hence to the extremity of the tail have no material 
change, excepting that the ridge formed by the union of the two 
tubercles, which at first stretches across the body obliquely, at 
length becomes horizontal. There are also two ridges across the 
bottom. 
The ridges on the side appear to have supported those horizontal 
processes, which will be better seen in the caudal vertebras of the 
plesiosaurus as analogous bones are usually found lying by the side 
of these vertebras. The two inferior ridges probably supported the 
fork of the chevron-shaped bone which occurs in the tail of all the 
lizard tribe, though we have not yet actually seen it in this animal. 
The character of these vertebrae is represented by the forty-sixth of 
those from Col. Birch’s specimen, and by one from the extremity 
of the tail exhibiting the annular part attached, belonging to Mr. 
Johnson. 
VOL. V. 4 D 
