416 PROF. OWEN ON AN ANOMODONT EEPTILE PEOM 



As in this Monotreme, the fossil shows an anterior articular sur- 

 face («, a') indicative of a more advanced " episternal " element. The 

 present anterior " sterneber " is 6 inches in length and the same in 

 breadth ; the average thickness may be put at j an inch, increasing 

 at the articular parts of the margin, of which there are four, the 

 lateral ones (b,W) being the broadest, swelling anteriorly to a thick- 

 ness of 1| inch. The swelling in the lateral (6, h') and posterior (c, c') 

 joints is towards the outer surface of the bone ; that of the anterior 

 joint (a a') turns towards the inner one. 



The outer surface is feebly concave between the lateral joints, and 

 also between each of these and the hinder joint ; transversely, below 

 the lateral joints, it is as feebly convex ; the surface is smooth, with 

 faint lines radiating towards the periphery. 



The inner surface is feebly concave both lengthwise and across ; 

 it is traversed by a low median narrow rising, hardly to be called a 

 ridge, which is thickest and most prominent at the anterior joint 

 (»') and subsides before reaching the posterior one. This latter sur- 

 face indicates the abrupt diminution of size, especially breadth, of 

 the second sternal bone which had been articulated therewith. 



I conclude from the character of that surface that such continuation 

 of a sternal series was present, as in the Chamaeleons and Seines, and 

 that the hinder surface (c, c', fig. 5) gave partial attachment to a pair 

 of sternal ribs, not total as in Varanus and Iguana. The lateral 

 articulation {h, h') shows two convex surfaces divided by an oblique 

 groove ; the lower and larger surface probably gave attachment to 

 a sternal rib, as in Ornithorhynchus (fig. 6); the upper, narrower 

 and longer one may have joined the coracoid, as in Monotremes. 

 The relative size of the foremost sternal bone, if followed by a 

 second of the size indicated by the joint-surface with the first, 

 would offer a Monotrematous character, but one of an importance 

 secondary to that which would be afforded if the anterior articular 

 surface of the large sternal bone supported an episternal element of 

 the T-shaped character shown in both Ornithorhynchus and Echidna, 

 as well as in Ichthyosaurus and most modem Lacertians. 



Such is the character of that bone in the labyrinthodont fossil 

 transmitted, under the name Batrachosaurus, by Mr. A. G. Bain to 

 the British Museum, and subsequently called Saurostemon by Prof. 

 Huxley * ; also in the specimen described and figured by me in the 

 ' Catalogue ' of Mr. Bain's fossils, on which I was at that time en- 

 gaged t, and in which it is noted that " the episternum shows the 

 clavicular groove on each transverse branch, as in that of Labyrin- 

 ihodon leptognathus":^. This groove for the reception of the clavicles 

 is more feebly marked in Ornithorhynchus, and is obliterated by con- 

 fiuence of the clavicles therewith in old animals, as in the specimen 

 figured by Clift to show the resemblance of the scapular arch in that 

 mammal to the same part of the skeleton in Ichthyosaurus §. 



* Geological Magazine, vol. v. 1868, p. 201, pi. xi. fig. 1, c. 

 t ' Catalogue ' &c., 4to, 1876, pi. Ixx. fig. 3. | Op. cit. p. 69. 



§ " As this form of the sternum (in Ichthyosaurus) appeared at the same 

 time quite new, I was very anxious not to fall into an error, and was reexamin- 



