With Figures 6 and 7.
The skeleton, as is commonly the case in Amphibia, retains a large amount of cartilage, in the deeper portions of which much calcareous matter is deposited, but not in the form of bone.
After Professor W. Kitchen Parker. The cartilage is dotted; the cartilage bones are shaded with oblique lines, and the membrane bones left white. The lettering is explained in the text.
The skull is remarkably flat, and the cranium proper of small size, though the total breadth is great owing to the large size of the orbital fossae. Its parts may be readily identified with the help of Figs. 6 and 7. There are two condyles (O. c.) formed by the two exoccipitals (E. O.) which are pierced each by a foramen for the vagus nerve (X). The otic capsules project laterally but contain only one ossification, the pro-otic (Pr. O.) which is pierced by the fifth nerve (V). The cranium in front of the otic capsules is cartilaginous and the cartilage is pierced by the optic nerve (//), but most anteriorly of all it contains a bone, the girdle bone, os en ceinture, or sphen-ethmoid (Sp. Et.). This bone has an internal vertical septum. On the upper surface of the cranium lie the fronto-parietals (P.F.): on its under surface the large para-sphenoid (P. S.), both developed in membrane. Other membrane bones are the two nasals (Na.): the two praemaxillae (P. Mx.) and the two maxillae (Mx.) which carry teeth in a simple series: and the quadrato-jugals (Qu. f.) which continue the line of the maxillae back to the distal ends of the quadrate cartilage (Qu.). The apertures of the external nares (E. n.) lie just in front of the nasals.
On the roof of the mouth, in front of the sphen-ethmoid, are the two dentigerous vomers ( Vo.), and at their anterior ends are the internal nares (I. n.). The palatine bone (Pa.), an investing membrane bone, is here placed transversely - a rare position seen again in Ichthyosaurus: while the pterygoid (Pt) lies parallel with the cranial axis. The latter is deeply forked behind: the outer process underlies the quadrate cartilage: the inner or pedicle is continuous with the quadrate and articulates with the ear-capsule. The squamosal (Sq.) is seen in the lateral view. It is the homologue probably of the squamosal + the praeopercular of bony Ganoids, two bones actually fused in Polypterus. The complex stapes (St.) or columella auris, the homologue of the Perch's hyomandibular; the cartilage ring or annulus tympanicus (A. T.), the homologue of the Ray's spiracular cartilage (?), which gives support to the tympanic membrane, and the hyoid apparatus are not present in this specimen (see p. 81, infra). The lower jaw consists of the two typical rami. The bulk of each ramus consists of an unaltered Meckel's cartilage (Mck.)y to which there is added distally an ossification known as the mento-meckelian (M.Mck.) formed by the ossification of the lower labial cartilages.
Meckel's cartilage is covered in part by two membrane bones, one on the outer side for a short distance of its distal part, the non-dentigerous dentary (D.): the other on the inner side, the articular (At.) as it is termed by Professor W. K. Parker, but identified formerly by Professor Huxley as a representative of the angular, coronary and splenial elements of the Sauropsidan lower jaw.
The vertebral column numbers nine vertebrae and a urostyle. The first vertebra is the only cervical vertebra: it is ring-like with a shallow centrum, two concave facets for the occipital condyles and two post-zygapophyses. The centra of the second to the seventh vertebra inclusive are pro-coelous. The centrum of the eighth is amphi-coelous, and of the ninth or sacral vertebrae biconvex anteriorly, the posterior surface being divided into two facets for the articulation of the urostyle. The neural arches are thin and narrow centrally, leaving in this region interspaces, one between the other. The articulating processes or zygapophyses are flat, and disposed normally. All the vertebrae except the first have lateral processes tipped with cartilage and varying in length, size, and direction. These appear to represent ribs fused to transverse processes. The urostyle is long and curved. Its posterior end lies just above the cloacal aperture. It is pierced laterally at its anterior end by the tenth pair of spinal nerves, and lodges the posterior part of the spinal cord.
The neural canal is often open terminally.
The shoulder-girdle is complete: and the glenoid cavity divides it into a dorsal scapular, and a ventral coraco-clavicular, moiety. The former consists of a broad semicartilaginous supra-scapula and an ossified scapula, the latter of an anterior clavicular bar separated by a fenestra from a posterior coracoid. Both clavicle and coracoid are connected by cartilage medianly to one another and to the corresponding parts on the opposite side. In the middle line project (1) anteriorly a conical bone ending in a cartilage plate - the episternum or anterior part of the interclavicle: (2) posteriorly a flattish bone with a cartilage disc, the hypo-sternum.
The pelvis has the V-shape characteristic of Anura or tail-less Amphibia. The long ilium trends backwards to a disc-like symphysis into which it enters, and which is partly cartilaginous and partly formed by the two ischia. The acetabulum is a deep cup.
The fore-limb consists of a humerus, and of a radius and ulna fused together, a furrow indicating externally the line of union. The elements of the carpus are small and consist in the proximal row of a scaphoid ( = radiale) and a cuneiform ( = ulnare): of a centrale displaced to the radial side: and in the distal row of a trapezium ( = carpale 1) carrying the thumb, represented by a single bone: a trapezoid ( = cp. 2) carrying the second digit, which in the male enlarges at the breeding season, and a single element ( = cp. 3, 4, and 5?) carrying the remaining three digits. The long hind-limb consists of a femur, and of a fused tibia and fibula. A long astragalus ( = tibiale) and calcaneum ( = fibulare) united at each end form the proximal row of the tarsus. The distal row consists of a tarsal element which carries a single bone, the extra digit or sixth toe; of a fused meso-and ecto-cuneiform ( = tarsalia, % and 3), while the ento-cuneiform (tarsale 1) and the cuboid (t. 4 and 5) are represented by ligament.