Wyman.] • 246 [June 5, 



explained. Mr. L. D. Eniei-soii, a miiiLng engineer, ivbo examined 

 these points Avith me, for a long time resident of Ontonagon County, 

 tells me he has observed a similar state of things at the junction of the 

 trap and the sandstone at Forest Falls and in a southerly direction 

 from Minnesota mine on the south boundary of the range, and that 

 he found there the sandstone beds resting unconformably upon the 

 beds of trap dipping north. 



On Symmetry and Homology in Limbs. By Dr. J. Wyman. 



Anatomists who have compared the fore and hind limbs of man 

 and animals, have mostly described them as if they Avere parallel rep- 

 etitions of each other, just as are any two ribs on the same side of the 

 body. By a few they have been studied as symmetrical parts, repeat- 

 ing each other in a reversed manner from befoi'e backwards, as right 

 and left parts do from side to side.* We have adopted this last mode 

 of viewing them, because, though open to grave objections, as will be 

 seen further on, the difliculties met with are, on the whole, fewer than 

 in the other, and because too, it is supported by the indications of 

 fore and hind symmetry in other parts of the body. 



Among animals, two organs or parts, generally speaking, are said to 

 be symmetrical when they are situated on opposite sides of an axis, 

 and are alike in form and size, but one is the reverse of the other, as 

 is everywhere obvious in those which are right and left. It is not to 

 be vmderstood, however, that this likeness is absolute ; for while it is 

 very generally true that such right and left parts are alike in size and 

 form, or very nearly so, it occasionally happens that they are very un- 

 like in these respects, still retaining, however, a certain amount of 

 symmetry. We have striking illustrations of this in the claws of many 

 Crustaceans, as in Astacus and Gelasimus, and in the right and left 

 halves of the body of Bopijrus. Among Acephalous Molluscs, this 



*Tlie following are among tlie more recent articles in which the homologies of 

 the limbs nnd their symmetry are treated of at length. 



S. R. Pittard. Article Symmetry— Cyclopadia of Anatomy and Physiology, Vol. 

 IV, p. 845. 



Observations on the Limbs of Vertebrate Animals. By George Humphrey, M. D., 

 r. R. S., etc.. Cambridge, 1860. 



Charles Martins. Nouvelle Comparaison desMembres. Memoires de I'Acad. des 

 SciencesdeMoutpellierT.il, p. 461. 1857. 



Also by the same author, Memoire sur I'OsteoIogie Compar^e des Articulations 

 du Coude et du Genou. Ann. des Sciences Naturelles. T. xviii. 4'°'=s6rie. 1862. 



Homologies des Membres Pelviens et Thoraciques de I'Homme, par le Docteur 

 Foltz. Journal de I'hysiologie, T. vi, 1863, p. 49. 



On Morphology and Teleolog}', especially in the limbs of Mammalia. By Burt 

 G. Wilder, S. B., Cambridge, 1865. From the Memoirs of the Boston Society of 

 Natural History. 



Also by the same author. Morphological Value and Relations of the Human Hand. 

 Am. Journal of Science, Vol. xliv, July, 1867, p. 44. 



I 



