The ribs, wliich articulate witli tlie dorsal vertebras by means of a transverse process like those of
most saurians, are eighteen on each side; tho anterior eight are true ribs the remaining one’s Coating.
Tliere are besides tliese eight stcriio-costal arc’s.
T he T rue R ibs. Ttio true ribs are known by their articular e.vtremiiies. They are tliick above
and have a hollowed articular head, a neck, a tubercle for the attaclimcnt of ligament and a body.
Tliey proceed downwards to behind and liave their anterior portion convex and smooth, tlieir internal
and posterior one hollowed and tlieir outer edge rounded and oven. The inferior extremity of the true rib
is hollowed to receive the superior part of the inter-articular ribs. They are nearly of the same length,
gradually decreasing in size from the first.
The floating ribs liavc, as their name implies, no intor-articulation with one another. Their
extremities arc lengthened into a point with exceedingly sharp edges ; by this character they are known
at once from the true ribs.
T he Sterxo-costal Arcs. They are eiglit:—each of them composed of seven bones—a ventral
rib and six inter-articular one's. The ventral ribs, tlie first of which lias connexion with the scapula’s,
are rounded at the centre and gradually diminish until their superior portion ends in a point. The
inler-arlieulai-s are mucli less thick than the venti-al ribs and liave like them an acute teiTnination
inferiorly; their upper portion is hooked, convex and articulates with the inferior part of the true ribs.
THE PELVIS.
: bones compose the pelvis:—above As in the Ichthyosauri s: the ilii, below nnd behind the ischii
and anteriorly the pubes.
T he I l ii. b. Tho ilii are the smallest bones of the pelvis and somewliat resemble that of tho
crocodile. They are oblong with a superior articular portion slightly extended, an inferior thickened
one, an outer conve.v and an inner concave surface and two thinned edges. The superior articular
portion is connected by cartilage a t the transverec processes of the two last dorsals and is roughened, tlio
inferior articidar e.xtremity composes nearly a tliird of the acetabulum and is likewise rough.
Connexion. Superiorly by arthrodia with the two last dorsal vertebræ, (which may be designated
sacral) inferiorly with the isctiii.
T he I schii. c. These bones placed at the posterior part of the pelvic region have a superior
articular extremity, a rounded neck, a descending flattened body and an inferior articular end. The
superior articidar extremity, which forms the middle part of the femoral concavity, is roughened and
triangular upon its anterior and posterior sides. The descending flattened body advances forwards and
terminates in a thin articular apophysis; posteriorly it is rounded.
Conne.tion. Below with one another, before with the pubes and above with tlie ilii and pubes.
T he P ubes, d. The pubes are large, flat bones, having an anterior convex edge, a median
symphysis, a posterior semi-lunar border wlircli composes witli those of the ischii tlie oval foramina, and
an Older ariicidarportion. Tlicir external surface is convex and smooth, thoir internal one concave.
Connexion. Before with one another, behind with tlie isciiii and without to the articular extremity
of the ischii. The pubes complete the cotyloid cavities.
T H E EX T R EM IT IE S .
The extremities were immensely long to obviate the great weight and the solidity of tlie skeleton, as
the Plesiosaurus was too lacertan to have tlic subcutaneous fat by which the mammalia arc buoyed up.
The anterior extremities have connexion with the trunk by means of the scapulæ and clavicles. Each of
them is composed of the shoulder, arm, fore-arm, and paddle.
THE SHOULDER.
The shoulder consists of two bones, the scapula and clavicle.
The Scai'ula. e. This large flat bono situated at the median region of the chest has an upper
articidar portion, a rounded neck, and an anterior and a posterior wide surface. The superior
articular portion of the scapula is rounded behind and is joined by synchondrosis to the internal part of
the head of the clavicle, from this articular portion the bone proceeds upwards and behind, forming the
large curve which distinguishes the superior part of the posterior surface of tliis bone. In the same
manner a curve is produced anteriorly but of a much loss extensive sweep. The outer aspect of tlic
scapula is marked in the centre by a considerable uprising, in consequence of wliicb it is rendered
irregularly convex anti in some places concave and flat.
Conne.xion. In the middle with one another, before with the sternum and clavicles, behind with the
anterior ventral rib, and superiorly with the clavicle and lunncriis.
Wc figure in the twenty-sixtli plate the scapula of tlic large individual found at Marsh’s Elm, which
difTere in shape from that of the Triatarsostinus,
T he Clavicle. / . Tlio clavicle is a long bone with a thickened articular end posteriorly, a body
awidened thin anterior portion. Its thickened articular end is flattened without and within
and has above a rounded side and below a concave one. The anterior widened portion is curved above
and sliglitly grooved; it supports the two lateral articulatory surfaces of the sternum.
Conne.xion. Before with the sternum and anterior flattened surfaces of the scapula and behind lo
the articular extremity of tho scapula, thus completing the glenoid cavity for the humerus.
Tlie clavicles found in plate twenty-five totally differ from those of the Triatarsostinus in shape.
Of the Arm. g. The humerus is long, curved and strong. It lias a head, a body and an inferior
extremity. The head of the humerus is large, round and deeply marked with many fossæ and eminences.
Aronnd this irregular surface the bone presents a roughened ridge for the capsular ligament. The body
is flattened and convex from side to side and upon its upper-thiixl is a tubercle for the attachment of
muscles. It is concave and thin behind, before rounded and convex throughout its long diameter. The
inferior extremity is wider and flatter than any other part and is divided into an anterior and a posterior
portion by a depression in its centre, which stretches fortli in a sharp point between the radius and ulna.
Connexion. Anteriorly and above witli the scapula and clavicle; inferiorly willi the radius and ulna.
Op the Eore-arm. The radius (h) \\o.s an upper an under flattened surface, a si/perior and
an inferior articular extremity, an external convex and rounded boundary and aji anterior and internal
semi-lunar edge.
Conne.xion. Above witli the liiimerus and below with the external and middle bone of the carpus
Tlic ulna (i) has a scvii-lunar edge posteriorly like tho radius liut it is not so mncIi curved, it is a
rounded lionc with its greatest diameter from above to below, and has an anterior convex border and an
upper and under articular surface.
Connexion. Above to the humerus and below by cartilage to the pisiform bone.
THE ANTERIOR PADDLE.
T he Carpus. Two longitudinal r g three bones form tlie carpus. We call them,
n the Ichthyosaurus, scaphoid, cuneiform and pisiform above; trapezium, trapezoid and unciform below.
The scaphoid bone is nearly circular; its upper and under surfaces are flat and smooth and its
edges plain. It is in close juxta-position with the outer edge of the cuneiform bone and its inferior
portion articulates with the trapezium.
The cuneiform bone by chance justifies the name wo give it and has, like all the bones of tiie carpus,
a flat upper and under surface and plain edges. Its wedge-like portion is insinuated between the radius
and ulna; it articulates rvitli the scaphoid without, below with tlie trapezium and wittiin to the pisiform
Iwne.
The pisiforrais is smaller tlian tlie two just described and has its long axis from side to side. It
articulates with the ulna above, below to the unciform bone and witliin to the trapezoid.
The trapezium, the first bone of the digital row, is a round, circular bone, having upon its superior
portion an articular junction with the scaphoid and cuneiform bones.