94 



NATURE 



\_May 26, 1887 



trace the History of the Evolution of Mammalian Teeth in 

 general." By Oldfield Thomas, British Museum (Natural 

 History). Communicated by Dr. Albert Giinther, F. R.S. 



The true homologies of the different teeth in the Marsupialia, 

 and especially in the Dasyuridce, have long been in a state of 

 confusion, a confusion that has been chiefly in regard to the 

 premolars, of which some members of the family have two, 

 others three, while generalized Placentals have four, and it is 

 therefore necessary to prove which teeth have been successively 

 lost in order to find out the- correct homologies of the remainder. 



Firstly, as to which of the three premolars of ordinary Marsu- 

 pialia has been lost in Dasyurus, with only two, it is shown that 

 it is the last premolar, or pm.*, that is missing in this genus. 



Next, it was necessary to find out which of the original four 

 premolars had disappeared in the ordinary three-toothed genera 

 of the Polyprotodonts, and this has been able to be done by the 

 fortunate discovery of a specimen of Phascologale in which there 

 are four premolars on one side, the additional tooth being 

 inserted behind the first premolar. The missing premolar is 

 therefore pm.^, the resulting premolar formula of Phascologale 



I • o ■ 3 • 4 

 1.0.3.4 



and of Dasyurus 



and Thylacinus being P.M. 



P.M. — . 



1.0.3.0 



The milk dentition in several of the Dasyurida is then 

 described, and also that of the Mesozoic Triacanthodon 'serrula 

 ■ (Owen), which is definitely proved to have a true Marsupial 

 milk dentition. 



An attempt is then made to trace out the history of the 

 evolution of mammalian teeth in general, and it is suggested 

 that the process by which a milk tooth was developed consisted 

 of two stages, firstly, a preliminary retardation of the permanent 

 tooth, and secondly, of the development of a temporary tooth in 

 the gap in the tooth-row caused thereby ; the retardation in the 

 first case being useful for packing purposes in a large-toothed 

 animal, while in a small-toothed form the same retardation, if 

 present by inheritance, would cause a more or less disadvant- 

 ageous gap, best filled by the assumption of a milk tooth. 



Following out this idea, it is shown how easily the transition 

 from the JMetatherian to the Eutherian state of tooth-change 

 may have taken place, a transition by the help of which a 

 complete series of diagrams can be drawn up, following the 

 history of each individual tooth, from the dentition of the earliest 

 mammals, homodont and monophyodont, down to the varied 

 forms of dentition, heterodont and diphyodont, existing at the 

 present day. 



For the Edentates alone it is necessary to draw up a special 

 branch of tooth development arising directly from the Proto- 

 theria, a branch for which the name of " Paratherian " is pro- 

 posed. 



Physical Society, May 14.— Prpf. W. E. Ayrton, Vice- 

 President, in the chair. — Mr. T. Mather was elected a member 

 of the Society. — The following papers were read : — On a modifi- 

 cation of a method of Maxwell's for measuring the coefficient 

 of self-induction, by Mr. E. C. Rimington. The method referred 

 to is given in Maxwell's " Electricity and Magnetism," § 778, 

 vol. ii., and is called "comparison of the electro-static capacity 

 of a condenser with the electro-magnetic capacity of a coil." The 

 apparatus used consists of a Wheatstone's bridge having the coil 

 in one, and the condenser as a shunt to the opposite, arm. In 

 order that no deflection may be produced, either for steady or 

 unsteady currents, a troublesome double adjustment of the resist- 

 ances is necessary, and to obviate this the modification was de- 

 vised. It consists in placing the condenser as a shunt to only 

 part of the arm, and this part can be varied by sliding contacts 

 without altering the whole resistance of the arm. An ordinary 

 resistance balance for steady currents is first obtained, and the 

 sliders are then adjusted until no deflection is produced on break- 

 ing the battery circuit. Under these circumstances it is shown 



that L = Kr2— , where K is the capacity of the condenser, r 



the resistance between the sliders, and D and B the resistances 

 of the arms in which the coil and condenser are placed. The 

 conditions of maximum sensibility are investigated, and also 

 those under which a telephone may replace the galvanometer ; 

 in the latter it is shown that the only possible solution is when 

 r = B, i.e. Maxwell's arrangement. The author believes his 

 modification would be made much more sensitive by adopting 

 the "cumulative" method used by Profs. Ayrton and Perry in 



their secohmmeter ; and in his case neither the speed nor the 

 " lead " need be known. Mr. W. N. Shaw asked whether any 

 serious difficulties were experienced with telephones, owing to 

 electro-static capacities of wires, &c., and Mr. W. E. Sumpner 

 pointed out that the particular arrangement given in Maxwell is 

 not always the most sensitive, as \\ as shown in his remarks at 

 the last meeting of the Society of Telegraph-Engineers. Mr. 

 Bosanquet thought the method a valuable one, and hoped many 

 experiments would be made on coils whose coefficients were 

 calculable, in order to find out the differences between calculated 

 and observed results. Prof. Ayrton referred to the paper 

 by Prof. J. J. Thomson in the Philosophical Transaction*, and 

 pointed out that the formula there given for the capacity of a 

 condenser in electro-magnetic measure, is identical with that 

 given in Maxwell, § 776, when the printer's error of in- 

 terchanging a and a in the denominator for Rg is corrected. — 

 On the production of sudden changes in the torsion of a wire 

 by change of temperature, by Mr. R. H. M. Bosanquet. A very 

 fine hard-drawn platinum wire, four or five feet long, was used 

 as a suspeusion for a ballistic galvanometer, and exhibited pecu- 

 liar phenomena. The steel needles were replaced by brass ones, 

 and the peculiarities investigated. When the room was warmed, 

 the needles swung round nearly 70° f r a few degrees rise of 

 temperature, and remained in about the same position for 

 further rises. If it was now cooled a few degrees (3" or 4° F.), 

 they quickly returned to their initial position. The author has 

 not found a complete explanation, but believes it to be due to 

 unequal expansion, and loose contact amongst molecules, and has 

 devised a simple mechanism to illustrate his meaning. Remarks 

 and suggestions were made by Prof. Perry, Mr. Lant Carpenter, 

 and the Chairman. — On a magnetic potentiometer, by Mr. A. P. 

 Chattock, read by Prof. Reinold. The "so-called" magnetic 

 resistance between two points on a magnetic circuit may be 

 expressed as the ratio of the difference of potential to the total 

 induction passing from one to the other (provided th«re be no 

 magnetomotive force between them). From the fact that the 

 volume integral of induction through a wire helix of constant 

 cross-section is proportional to the average difference of potential 

 between its ends, it follows that any alteration in that difference 

 of potential will give rise to an E.M.F. in the helix proportional 

 to that alteration. Hence, if the wire be connected to a ballistic 

 galvanometer, the combination may be called a magnetic poten- 

 tiometer. A helix is formed by winding wire uniformly on a 

 piece of solid india-rubber, or canvas gas-tubing, of constant 

 cross-section, using an even number of layers to avoid external 

 inductive effects, and leaving a small space between the turns so 

 as to allow the tube to bend without elongating. Experiments 

 made to measure the difference of potential between the ends 

 of a magnet gave satisfactory results. One end of the helix 

 was held stationary at one end of the magnet, whilst the other 

 was moved quickly to the other end of the magnet, 'and the re- 

 sulting throw of the galvanometer observed. This was next 

 done at two operations, and the sum of the two throws was very 

 nearly equal to the first. The results can be reduced to absolute 

 measure by passing the helix through a coil of n turns, bringing 

 its ends together, and starting or stopping a current, C, in the 

 coil, the resulting throw of the galvanometer being noticed. The 

 magnetomotive force used in this experiment is 47r«C. An 

 interesting discussion followed, in which Prof Perry, Mr. Shaw, 

 Prof. Ayrton, and Mr. Bosanquet took part, the latter mentioning 

 a measurement of magnetic potential made by himself some 

 years ago. — In consequence of the absence of Prof. S. P. 

 Thompson, his paper on secondary generators was postponed till 

 next meeting. 



Geological Society, May 11.— Prof. J. W. Judd, F.R.S., 

 President, in the chair. — The following communications were 

 read : — Further observations on Hyperodapedon gordoni, by 

 Prof. T. H. Huxley, F. R. S. The author briefly noticed the 

 circumstances under which he first described the occurrence of 

 Lacertilian and Crocodilian fossils in the Elgin sandstones, and 

 the confirmation which his views as to the Mesozoic age of these 

 remains had received from the discovery of Hyperodapedon in 

 English Triassic rocks and in India. The original type of 

 Hyperodapedon gordoni from Elgin wa«, however, in bad con- 

 dition, and the receipt at the British Museum of a second much 

 better preserved skeleton, found in the Lossiemouth quarries of 

 the same neighbourhood, had enabled him to add considerably 

 to the known characters of the genus, and to compare it more 

 thoroughly both with the recent Sphenodon (or Hatteria) of New 

 Zealand and with the Triassic Rhynchosaurus ariiceps, several 



