Oct. I, 1874] 



NATURE 



449 



THE BRITISH ASSOCIATION 



Reports 

 lalmlar Vieiu of the Classification of the Labyrinthodonta, by 



■ L. C. Miall. Summary of the Second Report on Labyrintho- 

 donta. 



AMPHIBIA 



LABVRINTHODONTA. 



■ A. — Centra of dorsal vertebnt discoidal. •— Genera I to 25. 



I. — EuGLYPTA. Cranial bones strongly sculptured. Lyra 

 I conspicuous. Mandible with well-developed post-articular pro- 

 ; cess. Teeth conical ; their internal structure complex ; dentine 



much folded, ralato-vomerine tuslcs in series with small teeth. 



Short inner series of mandibular teeth. Sculptured thoracic 



■ plates, with rellected process upon the external boider. 

 I * ]\ilatii!e foramina large, approximated. 



1 + Afaiidilile -with an internal articular buttress. 



X Orbits central or posteiior. 



1. Mastodonsaurus, Jagcr. 



2. Capitosaurus, Munst. 



3. Pachygonia, Huxley (?). 



4. Eurosaurus, D'Eichwald (?). 



5. Trematosaurus, Braun. 



6. Gonioglyptus, Huxley. 

 X X Orbits anterior. 



\ 7. Metopias, Von Meyer. 



^ 8. Labyrinthodon, Owen. - 



\ + + Mandible loithont internal articular buttress, 



J 9. DiadetO;;nathus, Miall. 



* * Palatine foramina small, distant. 



10. Dasyceps, Huxley. 



11. Anthracosaurus, Huxley. 



" H. — Brachyopida. Skull parabolic. Orbits oval, central 

 ". or anterior. Post-articular process of mandible wanting (?). 



12. Brachyops, Owen. 



1 13. Micropholis, Huxley. 



; 14. Rhinosaurus, Waldheim. 



1 15. Bothriceps, Huxley. 



i~ HI. — Malacocvi.a. Skull vaulted, triangular, with large 

 I postero-Iateral expansions. Lyra consisting of two nearly 

 e' straight longitudinal grooves, continued backwards as ridges. 

 ' Orbits large, posterior, irregular. Temporal depressions, passing 

 'backwards from orbits. No post-articular process to mandible.^ 

 1 * 'J'eet/i -oith large anterior aiul posterior cutting edges. 



16. Loxomma, Huxley. 



* * Teeth conical. 



17. Zygosaurus, D'Eichwald. 

 J IV. — Athroodonta. Maxillary teeth wanting. Vomerine 

 1 teeth aggregated. Orbit imperfect. 



iS. Batrachiderfieton, Hancock and Atthey. 

 19. rteroplax, Hancock and Atthey.'' 

 [V. — An uncharacterisid group for the reception of some or 

 ( all of the following genera.] 

 j 20. I'holidogaster, Huxley. 



i 21. Ichthyerpeton, Huxley. 



) 22. rholiderpeton, Huxley. 



23. Erpefocephalus, Huxley. 



VI. — ArciiegijSAi;ria. Von Meyer. Vertebral column 

 notochordal. Occipital condyles unossified. 



24. Archegosaurus, Goldfuss. 



25. Apateon, •' Von Meyer. 



B. — Centra of dorsal vertebra elongate, contracted in the midale. 



VII. — IIeleotpirepta. Skull triangular, with produced, 

 tapering snout. Orbits central. Mandibular symphysis very 

 long, about one- third of the length of the skull. 



26. Lepterpeton, Huxley. 



VIII. — Nectkide.\. Epiotic cotnua much produced. Supe- 

 rior and inferior processes of caudal vtrtebr.-e dilated at the ex- 

 tremities and pectinate. 



27. Urocordylus, Hu;;ley. 



28. Keraterpeton, Huxley. 



I IX. — Aistopoda. Limbs wanting. 



29. Ophiderpeton, Huxley. 



30. Dolichosoma, Huxley. 



* This cliaracter is not of primary importance, but secm.s to be avaiUItle 

 for an .irrangemcnt determined by other considerations. 



- Orbits unknown. 

 3 Loxomma. 



< The vomerine teeth arc unknown, and this genus may therefore require- 

 to be removed. 

 5 Of doubtful distinctness. 



X. — MiCROSAURiA, Dawson. Thoracic plates unknown. 

 Ossification of limb-bones incomplete. Dentine non-plicate, 

 pulp cavity large. 



31. Dendrerpeton, Owen. 



32. Hylonomus, Dawson. 



33. Hylerpeton, Owen. 



SECTIONAL PROCEEDINGS 

 SECTION A— Mathematics 



On the Photographic Operations connected with the coming 

 Transit of Venus, by Captain Abney, R.E., F. R.A.S. 



As is doubtless well known to all, there will be an applica- 

 tion of photography to register the passage of Venus across the 

 sun's disc, and it may not be amiss to give an outline of the 

 processes, cS:c., that will be adopted. It has been determined by 

 the Astronomer Royal that at every photographic station a photo- 

 graph shall be taken every two minutes during the transit, and it has 

 been a matter of considerable labour to work out a process that 

 will admit of such a large number of negatives being taken in a 

 hot climate. In Kerguelen's Land it would be perfectly feasible 

 to adopt the ordinary wet process, the low temperature admitting 

 o( it, but in a temperature of 90' F. the evaporation of the . 

 volatile constituents of the collodion would render such a 

 jirocedure inapplicable, as all practical photographers will admit. 

 In India, where I have worked extennvely, coating two or three 

 plates in succession in a large-sired tent has sometimes proved 

 injurious. With such experience I venture to think that it 

 would have been madness to trust to the wet method for four 

 hours, unless the conditions of personnel o{\\\z p.arties were con- 

 siderably altered. Sir G. Airy, after much anxious deliberation, 

 and with the advice (and tliat not hastily formed, by any means) 

 of Mr. De la Rue, determined to adopt a dry process if practicable. 

 After considerable experiments conducted at Chatham, it was 

 determined to adopt an albumen dry process, using a highly 

 bromised collodion, and strong alkaline development. There 

 were several advantages in this : — ( i ) At the critical time the 

 photographers would have nothing to distract their attention 

 excepting placmg the dry plates in the slide and developing every 

 twelfth plate exposed, in order to regulate the exposure ; (2) the 

 irradiation was much diminished by tire use of albumen, a point 

 of no small importance when measurements have to be taken ; 

 (3) the shrin'ivage of the film is reduced to zero when the plates 

 are properly prepared. 



In regard to the first advantage claimed, it will be apparent 

 that plates prepared at leisure will have a much superior advantage 

 to those prepared in the hurry of the moment as would be the 

 case with wet plates. The chances of stains and spots are dimi- 

 nished tenfold, and we may expect a much clearer picture. 



The true explanation of irradiation has been argued of late in 

 NAruRE, and perhaps I tn.ay be pardoned for dwelling an in- 

 stant on that point. Irradiation may be divided into two kinds, 

 viz., that occurring from reflection from the back of the plate, 

 and that occurring from reflection from the particles of bromide 

 or iodide of silver in the collodion film. The fir-st requires no 

 explanation. If a film be insufficiently dense and of such a 

 colour as will cut off the most active rays of the spectrum, no 

 irradiation on that account need be anticipated. Iodide of silver 

 fulfils this condition much more fully than does bromide of silver, 

 the former approaching to a yellow colour-, whilst the latter is 

 almost white. A thin layer of iodide is much more efficietit in 

 cuttirrg olT the blue end of the spectrum than is the bromide ; 

 hence, if irradiation through reflection from the back of the plate 

 is to be overcome, it is wise to use a certain proportion of iodide 

 iir the collodion. Practically I have foimd that in the dry pro- 

 cess under consideration, three parts of iodide to two of bromide 

 give the best results without diminishing the sensitiveness of the 

 film. The second cause of irradiation, viz., reflection from the 

 particles of bromide and iodide, is not hard to explain. When a 

 colloid body such as gelatine or albumen is brought in contact 

 with a soluble sa't of silver, the resisting compound is found to 

 be one which is singularly free from this defect. If a ray of light 

 be allowed to fall at right angles upon a very thin cell containing 

 an enrulsion of bromide of silver, the cell having worked glass 

 sides and ends, it will be found that the ray of light will be 

 scattered considerably, apparently in a logarithmic curve ; the 

 surface nearest the source of light wdl not be affected, but it will 

 spread from that surface towards the other, a physical line of 

 light becoming an area. If, however, a colloidal salt of silver 

 be introduced it will be found that this area is much diminished, 



