258 Sh' J. W. Daichon — On Hylonomus Lyelli. 



from tlie Tonga Islands appear to be of submarine formation. The 

 absence or presence in different strata of any sensible proportion of 

 calcareous matter and organic remains is perhaps related to tlie 

 more or less rapid rate of accumulation at different epochs of 

 eruption. The volcanic material ejected seems to have been almost 

 exclusively of fragmental character, and in some cases there are 

 indications of violent explosive action. This is quite in accord with 

 the andesitic nature of the materials thrown out, which are of types 

 common in the Pacific region. As to the age of the rocks it would 

 be idle to speak until the evidence of their organic contents has 

 been duly set forth, but it would undoubtedly be very rash to refer 

 them all to a Eecent age, and some of them may be found to go 

 back far into Tertiary times. 



IV. — Note on Hylonomus Ltelli, with Photogkaphio Eeprodtjc- 

 TioN of Skeleton. 



By Sir J.William Dawson, F.R.S., etc. 



(PLATE VIII.) 



AS a sequel to my recent paper on new specimens of Den- 

 drerpeton, I have thought it desirable to reproduce by 

 photogravure, for comparison, the type specimen of Hylonomus 

 Lyelli now in the collection of the Geological Society of London. 

 The reproduction (Plate VIII.) is of the natural size, though less 

 distinct than in the original. Though the bones are scattered, this 

 specimen enabled me, by measuring the separate bones and adding 

 the cuticular scales found on other specimens, to restore the animal 

 in my " Airbreathers of the Coal Period." ^ 



The specimen represented is one of the largest found. Most of 

 the others represent smaller (probably in some cases half-grown) 

 specimens, though not showing any structural differences. It will 

 be noticed that the caudal vertebra are seen in this example, a fact 

 which I had forgotten when the former paper was written. In the 

 species of Hylerpeton and Fritschia, though the teeth are different, 

 the development of the limb-bones seems to have been similar. In 

 Smilerpeton occidentatum the limbs would seem to have been shorter 

 than in the case of other forms in the erect trees, and the skull long 

 and narrow. 



The sj)ecimen here delineated, though the bones are scattered, has 

 the advantage of lying on a flat plane of lamination. Some of the 

 thinner bones have, however, scaled off, or have been removed by 

 aqueous infiltration. The cavities left by these have been touched 

 with white so as to bring them out. The difficulty in restoring 

 most of the specimens in the erect trees arises not from the absence 

 of the bones, but from these being scattered through non-laminated 

 material, sometimes soft and crumbling, in other cases hard and 

 arenaceous. In either case it is a work of time and care to uncover 

 the bones, and many of these cannot be reached without risking the 



^ Also on a larger scale in " The Chain of Life in Geological Time." 



