1883.] On the Skeleton of the Marsipobranch Fishes, 447 



The permeability of the system is then deduced from the comparison 

 of resistances in the air space of the core of the solenoid, and in the 

 air spaces which form the breaks in the magnetic circuit. 



The coefficient of electromagnetic momentum is then, by means of 

 known equivalents, expressed in terms of the permeability of the 

 system, and thence, according to the above result, in terms of the 

 configuration of the breaks in the magnetic circuit. 



A method is then developed of calculating the coefficient of electro- 

 magnetic momentum from the observed mean current during the 

 excitation of a tuning fork of known period. 



Four experiments of this description were made, and the value of 

 the terms depending on the configuration of the air spaces analysed 

 and interpreted in connexion with the numerical results thus obtained. 



In this manner I was led to distinguish two theoretical cases which 

 were connected by an empirical configuration formula, bridging over 

 the gap between them. The experiments in question agree with the 

 configuration formula to a degree far beyond what could have been 

 anticipated, considering the roughness of the methods employed for 

 the determination of the several elements concerned. 



The result is, that in a certain class of electromagnetic systems, 

 when the configuration is given, the permeability and coefficient of 

 electromagnetic momentum can be approximately assigned, and the 

 whole electromagnetic behaviour of the system approximately calcu- 

 lated. 



With this theory at my disposal I hope to make further contri- 

 butions to the knowledge of electromagnetic tuning forks. 



II. " On the Skeleton of the Marsipobranch Fishes. Part II. 

 The Lamprey." By W. K. Parker, F.R.S. Received 

 January 10, 1883. 



(Abstract.) 



In working out this type I have been greatly indebted to the 

 labours of J. Miiller, Huxley, Schneider, Balfour, and Scott. 



For materials I am indebted to two of the above-mentioned anato- 

 mists, namely, Professor Huxley and the late Professor Balfour, also 

 to Surgeon-Major Francis Day, of Cheltenham, and Osbert Salvin, 

 Esq., F.R.S. 



The transformed skeleton is described in various young individuals 

 of the Sea Lamprey (Petromyzon marinus), from four to eight or nine 

 inches in length. The smallest of these was scarcely through its meta- 

 morphosis. A specimen of P. planesi was worked out at the same 



