Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/805

Rh 761 excavated border, between which and the posterior margin of the sphen-ethmoid the cranial wall is tmossified. Throughout its wkole B S- Fio. 14. Tlie slcull of Epicrium glutinoswn. A, dorsal; B, ventral; C, lateral, view. The letters have the same signification as before. extent, this large ossification, which represents the exoccipitals, the elements of the periotic capsule, and the alisphenoids, is firmly ankylosed with the parasphenoid. There is a well-developed sphen- ethmoid, similar in its general characters to that of the frog. It is very closely united, if not ankylosed, with the vomers and pre- maxillae. The roof of the skull is completed by two parietals, two f rentals, and two large nasals, which unite in a long suture, except in front, where, for a short distance, they are separated by the ascending processes of the premaxillae. The dentigerous oral pro cesses of these bones are short, and unite by suture with the maxilla;. These send up broad plates which lie in front of and below the orbit, on the sides of the face. The canal for the suborbital tentacle per forates the maxilla in front of the orbit. Posteriorly, the maxilla unites with the squamosal, which is a broad plate firmly fixed to the quadrate, but somewhat loosely united with the frontal and parietal and with the complex occipito-otic bone. A small crescentic post-orbital bone (denoted by I in Fig. 14) articulates with the maxillary and squamosal, and with another bone (2), which answers very nearly to the prefrontal of a reptile. Between the nasal bone and the premaxilla, above and below, and the maxilla behind, a small bone (3) is fitted. The quadrate bone is repre sented by the ossification of the distal end of the suspensorium, which is inclined a little backwards. The stapes is large and well ossified. Two distinct ossifications, an angulo-articular and a dentary, are discernible in the mandible ; and the second short row of teeth, inside those of the dentary, seems to indicate the existence of a splenial element. In the LABYRINTHODONTA the skull presents the extremes of form which are met with among the Amphibia, from the elonga tion observable in Archegosaurus, to the short and broad form of Mctnpias and Brack-yaps. The chief characters by which the laby- rinthodont cranium differs from that of its existing allies are the following : The occipital condyles in some genera remained long, if not per manently, cartilaginous ; and one or two supra-occipital ossifica tions (probably membrane bones) very generally occur. The epiotics are prominent, and appear to remain permanently separate from the adjacent bones. In front of them, and articulated with the outer edges of the parietal and frontal, are two bones, which are com monly identified with the &quot;squamosal&quot; and &quot;post-frontal&quot; of the higher Vertebrata. The &quot; post-frontal&quot; articulates anteriorly with a Targe &quot;prefrontal&quot; bone, which bounds the dorsal and anterior contour of the orbit. The outer edges of the &quot;squamosal&quot; and &quot;post-frontal&quot; articulate with two bones, termed the &quot;post- orbital&quot; and the &quot;supra-temporal.&quot; The post-orbital lies in front of the other, and contributes to the posterior margin of the orbit, the rest of the contour of which, between the post-orbital and the prefrontal, is usually completed by a large jugal. Articulated, dorsally and internally, with the &quot;supra-temporal,&quot; and, anteri orly, with the jugal, is a &quot;quadrato-jugal,&quot; which sometimes ex tends into the articular surface for the lower jaw, and in some cases, at any rate, overlaid a quadrate ossification. There are long paired nasals, between the anterior ends of which the ascending processes of the premaxillae are received; and between these bones, the un usually long maxillae, and the prefrontals, distinct &quot;lachrymal&quot; ossifications occur. The vomers are large, meet in a long median suture, and bear teeth. The palatine bones, also dentigerous, bound the posterior nares in front, and are elongated antero-posteriorly. The mandible presents a dentary (probably including a splenial) clement, an angular and an articular ossification. Those surfaces of the cranial bones which were covered by the skin are usually rugose, and sculptured much in the same way as those of the crocodiles, and they frequently present symmetrically- disposed grooves, the so-called &quot;mucous canals,&quot; which, very pro bably, lodged sensory apparatuses resembling the similarly-named structures in fishes the homologucs of which are found in existing Urodela. The hyoid is unknown, and what appears to be traces of a branchial apparatus have been observed only in young specimens of Archego- saurus. Hence it is probable, not only that no known Labyrinth- odonts were perennibranchiate, but that the air-breathing condition supervened early in the course of their development. The Labyrinthodonts doubtless possessed a well- developed chon- drocranium, but such a structure would necessarily perish in the course of fossilisation. The singular resemblance of the labyrinth - odont skull to that of the Pcronida, in the arrangement of the bones which bound the cavity of the mouth, and the disposition of the teeth upon them, suggests a comparison of the other cranial bones in the two groups. Starting from the nasals of Epicrium, which may be safely identified with those of the Labyrinthodonts, the bone marked (2) in Epicrium corresponds very closely with the labyrinthodont &quot;prefrontal;&quot; and that numbered (1), with the &quot;post-orbital.&quot; No. 3 in Epicrium, in some respects answers to the so-called &quot;lachrymal&quot; of the Labyrinthodonts; while the maxilla of the Csecilian may be taken to represent both maxilla and jugal of the Labyrinthodont. But if this be so, the squamosal of Epicrium corresponds with the supra-temporal of the Labyrinth odont ; and a question arises as to the true nature of the &quot;squa mosal&quot; and &quot;post-frontal&quot; of the latter. 1 The Limbs. The pectoral arch in the Amphibia is distinguish able into a scapular, a coracoidal, and a pnecoracoidal region, although the extent to which these parts of the primitive cartilagin ous arch become separately ossified varies very much in the different members of the group. In Proteus, Menobranchus, Cryptolranchus, and Mcnopoma, ossification occurs only in the scapular region. In Siren and Amphiuma an additional broad coracoidal ossification occurs, but it does not meet the scapular ossification in the glenoidal cavity. The junction, however, takes place in Siredon and the Salamanders. In none of the Urodela does any ossification appear in or upon the prsecoracoidal or supra-scapular cartilage. A supra-scapular ossification exists in all known Anura. All but Microps and Hylccdactylus* have a pracoracoid, which acquires a sheath of bony matter. The glenoidal cavity is bounded by the prrecoracoid, corncoid, and scapula ; and in some cases (e.g., Dacty- Uthra) the ossified ends of the three unite and give rise to a tri- radiate suture in the glenoidal cavity, just as the pubis, ischium, and ilium of most Vertebrata unite in the acetabulum. In Systonia gibbosum, contrary to the usual rule, the prsecoracoid is far broader than the coracoid (Parker). In the higher Anura, a median piece, of very variable size, form, and consistency, extends forwards from the junction of the prae- coracoids. Mr Parker considers it to be an outgrowth from these, and terms it the omosternum. The long bones, both in the fore and hind limbs, consist of an axis of cartilage, sheathed in, and more or less replaced by, a dia- physis of membrane bone. The extremities of the cartilages fre quently undergo calcification, and are thus converted into epiphyses. A strong crest characterises the Kumerus in many male Anura. In the latter, the radius and ulna coalesce into one bone, while in all other Amphibia, they remain distinct. In Siredon, Cryptobranclms, and Menopoma, the carpus contains eight separate cartilages, of which three the radiale, intermedium, and ulnare form a proximal row ; and four distalia, a distal row. Between these two series lies a single ccntralc. In Menobranchus, there are only six carpal cartilages the ulnare and intermedium, and the radiale and radial distale, respectively, having apparently, as Gegenbaur suggests, coalesced. In Amphiuma didactylum, the number of carpal cartilages is reduced to four, and in Proteus to three. In both these cases the two largest cartilages form a proximal row. 1 On the structure of the skull, as of whatever else is known of the organisation of the Labyrinthodonts, the reader will find full, ex cellently arranged, and well-digested information in the &quot; Report of the Committee of the British Association on the Labyrinthodonts of the Coal Measures,&quot; drawn up by Mr Miall, British Association Reportt, 1873. 2 Parker On the Shoulder Girdle, p. 67. I. 06