240 REPORT — 1873. 



a Labyrinthoclont from the Northumberland coal -field, which Mr. T. P. Barkas 

 has permitted us to study, seems to exhibit the same feature *. 



The superior aud inferior spinous processes differ greatly as to length 

 and form. In Archegosaurus aud many others the spinous processes, both 

 superior and inferior, are broad and quadrilateral. In Urocordi/lus and 

 CEstocephalusf the superior and inferior spinous processes of the long tail are 

 elongate and fan-shaped, being dilated, compressed, and truncated at the 

 distal ends, so as to suggest great swimming-power. 



The inferior arches are rarely seen to advantage ; but in Archegosaui-us they 

 are large and complete, forming a spacious canal for the caudal vessels J. 



By study of young specimens oi Archegosaurus it has been ascertained that 

 the superior vertebral arches ossify before the inferior, and the anterior ver- 

 tebrae before the posterior. Von Meyer thinks it probable that the superior 

 arches were ossified to a considerable extent before the close of embryonic 



life §. 



The atlas of Mastodonsaurus has been figured and described ||. It is a 

 flattish disk, presenting two oval cavities to the occipital condyles, and nearly 

 smooth behind. Above, the lamiuae enclose the chief part of the spinal 

 foramen, ascending to form a spinous process of considerable but unknown 

 height. A cavity for the odontoid occupies nearly the centre of the bone, 

 between the articular facets, and communicates with the spinal foramen by a 

 constricted passage. 



Jlibs. — No Labyrinthodont is known to have been devoid of well-developed 

 ribs. They are generally attached to all the vertebrae in advance of the 

 pelvis, and in some cases, at least, are present in the anterior part of the 

 caudal region also. 



As to form, they are usually compressed (transversely to the axis of the 

 trunk) at either end, but are nearly cylindrical in the centre of the shaft. 

 They are short, relatively to the probable dimensions of the thorax, and 

 strongly curved. A capitnlum and tuberculum are present in aU well- 

 preserved examples. Both articular surfaces are slightly concave and 

 adjacent, and in most of the Labyrinthodonts both appear to have articulated 

 with the vertebral transverse process ; a notch or groove commonly separates 

 them, and is usually continued for some distance along the shaft of the rib. 



Sternal or abdominal ribs are not known to occur in this order. 



It appears from the extensive suite of specimens described by Von Meyer ^, 

 that the ribs of Archegosaurus were developed and partially ossified at a veiy 

 early period, perhaps before the close of embryonic life. Some very young 



* This fossil is named Macrosaurus ^^olyspondylus by Mr. Barkas ; but its generic 

 or specific distinctness cannot as yet be affirmed. 



t It is impossible not to suspect the identity of these genera. Prof. Cope remarks 

 (Trans. American Phil. Soc. vol. xiv. N. S. p. 16) :— " It [Oi.ifoccphalus.'] differs [from 

 Urocordylus] only in the presence of elongate lizard-like ribs, and in the absence of 

 ' oat-shaped scales ' of the lower surfaces." But Urocorch/lus has slender ribs, of more 

 than usual length. Were the absence of oat-shaped scutes from the ventral surface of the 

 American examples of (Estoccphalus established, little could be proved thereby. In the 

 Northumberland coal-field Labyrinthodonts abound, yet the scutes appear not to have 

 been hitherto discovered. On the following page of his ' Synopsis,' however, Prof. Cope 

 says of (Estocephalus: — "The skin has been occupied by a great number of closely packed, 

 curved, spine-shaped scales. They have occupied the ventral integument, passing from 

 the median line of the belly outwards and posteriorly, having acute tips, which may or 

 may not have penetrated the skin on each side." This structure cannot differ essentially 

 from the clieyron pattern of oat-shaj^cd scutes found in Urocordylus. 



X Von Meyer, 'Reptilien' &c., p. 107, t. xii. flg. 7. § Eeptilien &c., p. 29. 



II Paliiontologie Wiirtembergs, t. v. figs. 4, 5, and p. 07. 1[ Reptilien &c., p. 33. 



