Page:EB1911 - Volume 03.djvu/542

 sacrum, whilst in a few others the number of segments is still further reduced by the co-ossification of one or two vertebrae preceding that corresponding to the normal sacral and by the fusion of the two first vertebrae, the extreme of reduction being found in the genus Hymenochirus, the vertebral column of which is figured here (fig 6.)

As stated above in the definition of the order, the Stegocephalia have retained most of the cranial bones which are to be found in the Crossopterygian fishes, and it is worthy of note that the bones termed post-temporals may give attachment to a further bone so prolonged backwards as to suggest the probability of the skull being connected with the shoulder-girdle, as in most teleostome fishes. This supposition is supported by a specimen from the Lower Permian of Autun, determined as Actinodon frossardi, acquired in 1902 by the British Museum, which shows a bone, similar to the so-called “epiotic cornu” of the microsaurians, Ceraterpeton and Scincosaurus, to have the relations of the supra-cleithrum of fishes, thus confirming a suggestion made by C. W. Andrews (28). As in fishes also, the sensory canal system must have been highly developed on the skulls of many labyrinthodonts, and the impressions left by these canals have been utilized by morphologists for homologizing the various elements of the cranial roof with those of Crossopterygians. The pineal foramen, in the parietal bones, is as constantly present as it is absent in the other orders. Although not strictly forming part of the skull, allusion should be made here to the ring of sclerotic plates which has been found in many of the Stegocephalia, and which is only found elsewhere in a few Crossopterygian fishes as well as in many reptiles and birds.

In the orders which are still represented at the present day, the bones of the skull are reduced in number and the “primordial skull,” or chondrocranium (fig. 7), remains to a greater or less extent unossified, even in the adult. Huxley’s figures of the skull of a caccilian (Ichthyophis glutinosus), fig. 8, of a perennibranchiate urodele (Necturus maculosus＝Menobranchus lateralis), fig. 9, and of a frog (Rana esculenta), fig. 10, are here given for comparison.

The skull, in the Apoda, is remarkably solid and compact, and it possesses a postorbital or postfrontal bone (marked 1 in the figure) which does not exist in any of the other living batrachians. The squamosal bone is large and either in contact with the frontals and parietals or separated from them by a vacuity; the orbit is sometimes roofed over by bone. The presence, in some genera, of a second row of mandibular teeth seems to indicate the former existence of a splenial element, such as exists in Siren among the Caudata and apparently in the labyrinthodonts.

In the Caudata, the frontals remain likewise distinct from the parietals, whilst in the Ecaudata the two elements are fused into one, and in a few forms (Aglossa, some Pelobalidae) the paired condition of these bones has disappeared in the adult. Prefrontal bones are present in the Salamandridae and Amphiumidae, but absent (or fused with the nasals) in the other Caudata and in the Ecaudata. In most of the former the palatines fuse with the vomers, whilst they remain distinct, unless entirely lost, in the latter. The vomer is single, or absent, in the Aglossa. In the lower jaw of most of the Ecaudata the symphysial cartilages ossify separately from the dentary bones, forming the so-called mento-meckelian bones; but these symphysial bones, so distinct in the frog, are less so in the Hylidae and Bufonidae, almost indistinguishable in the Pelobatidae and Discoglossidae, whilst in the Aglossa they do not exist any more than in the other orders of batrachians.

No batrachian is known to possess an ossified azygous supra-occipital.

Although there are four branchial arches in all the larval forms of the three orders, and throughout life in the Sirenidae, the perennibranchiate Proteidae have only three (see fig. 11). In the adult Apoda these arches and the hyoid fuse into three transverse, curved or angular bones (see fig. 13), the two posterior disconnected from the hyoid. In the Ecaudata, as shown by F. Gaupp (29) and by W. G. Ridewood (30), the whole hyobranchial apparatus forms a cartilaginous continuum, and during metamorphosis the branchialia disappear without a trace. The hyoid of the adult frog (fig. 12)