ON THE STRUCTURE OF THE LABYRINTHOUONTS. 229 



parietals, originally i^aircd bones, are respectively united at that early period. 

 This rapid formation of a solid and compactly articulated skull does not pre- 

 clude tho further growth of every separate bone. In both Crocodiha and 

 Labyrinthodonts the skull ultimately becomes many times as large as it was 

 at birth, retaining all the time its accurately closed sutures, and increasing 

 by additions to all tho borders of each ossification. The growth of the Cro- 

 codilian skull appears to be quite indefinite, ending only with the life of the 

 individual ; and the same may have been true of the Labyrinthodont. This 

 mode of enlargement is compatible with great progressive changes in the 

 proportions of the skull. In Crocodilia and Labyrinthodonts alike, the face 

 increases more rapidly than the brain- case ; so that the orbits may recede 

 from near the centre to the junction of the posterior and middle thirds of the 

 skull. This is the case, for example, with Archegosaurus Decheni. 



All these peculiarities of the skull — the early 'ossification and junction by 

 suture of the cranial bones, their indefinite or, at least, protracted growth, the 

 generally persistent sutures which are implied thereby, the ever- increasing 

 ratio of the entire skull to the chamber in which the brain is lodged, and, 

 lastly, the pitted sculpture of the subcutaneous surfaces — are interesting 

 points of physiological resemblance between the Labyrinthodonts and Cro- 

 codilia ; but they are too directly associated with mode of life and external 

 conditions to support any argument as to zoological aflinity. 



The orbits vary much as to size, position, and form. In Loxomma they 

 are -36 of the length of the skull along the middle line ; in Basyeeps not 

 more than -1. In Mctoinns they lie in the anterior half of the skull; in 

 Mastodonsaurus they are nearly central; in Capitosaurus they lie in the 

 posterior half. As to form, they may be round, oval, elliptical, or irregular. 

 In Pterojylax and Batt-achiderpeton the outer bony wall (at least) of the orbit 

 seems to be deficient. 



The interorbital space and the external nasal apertures are equally variable. 



The Occijntal Segtneiit.— It is to he regretted that the occipital region of 

 the Labyrinthodonts, especially of the Carboniferous genera, is so imperfectly 

 known. No part of the skuU would yield characters of greater zoological 

 siguificance were its structure fully revealed. In most of the Carboniferous 

 examples examined nothing is shown of the occipital segment, except one or 

 two supraoccipital plates. The deficiency of occipital condyles in Archetjo- 

 saurus, of which many singularly perfect specimens have occurred, seems to 

 show that, like the vcrtebilil centra of that genus, they were never ossified, 

 but remained cartilaginous throughout hfe. Loxomma, on the contrary, 

 which has well-ossified centra, has also ossified condyles ; they are small, 

 very convex, and closely approximated. In the Triassic Labyrinthodonts the 

 occipital region was fully ossified; and these are our best guides to the 

 structure of the occipital segment in the whole order. Even in the Triassic 

 species the basioccipital is concealed by a parasphenoid, and the form of the 

 occiput, with its numerous cavities and processes, is not favourable to the 

 complete preservation of details. 



The boundaries of the component parts of the occipital segment have in no 

 case been traced. It is probable that in the Mastodonsauria (e. g. Tremato- 

 saurus) a pair of exoccipitals surrounded the foramen magnum*, and sup- 

 ported the occipital condyles, that a cartilaginous supraoccipital, ultimately 

 replaced by a pair of membrane-bones, surmounted the segmetot, and that in 

 the basioccipital tract the cartilaginous primordial skull was never ossified, 

 but was underlain and finally absorbed by the parasphenoid plate. In 

 * Burincister, Tronatosaums, p. 24. 



