1867.] 247 [TVyraan. 



difference is in some cases very remarkable, as in Ratliolites, Mono- 

 pleura, etc., ■where one of the valves is conical and spirally twisted, 

 while the other is quite flat, and in its relation to the other valve, re- 

 sembles the operenlum of certain Gastei'opods. 



Among Vertebrates such differences are much less frequently met 

 with ; they however exist, and symmetry of right and left parts, even 

 in the human skeleton, is not constant ; it may even be doubted 

 whether absolute symmetry exists anywhere. Attention has recently 

 been called to asymmetry in the base of the skull, and by a compari- 

 son of the bones of the lore arm in ten skeletons, we have found the 

 right ulna longest in eight, the left in one, and in one the right and 

 left bones were equal. Differences in length and weight were also 

 found in the clavicles, the humerus, and other parts of the skeleton, 

 but in different degrees. The close approach to absolute symmetry in 

 some of the minuter details of structure is sometimes quite remark- 

 able, as in the arrangement of the papillaj on the tips of the fingers 

 and toes of most individuals ; in some, however, the asymmetry of 

 these parts is quite marked, and in others even the pattern of the 

 figure on the two sides is changed. More marked instances of asym- 

 metry exist in the unequal nostrils, and more or less bent vomer of 

 some Cetaceans, in the rudimentary right, but immensely developed 

 left tusk of the Narwal, in the lower jaw of the adult male Mastodon 

 which has a tusk on the left side, but none on the right, in the unequal 

 development of the ovaries and oviducts of most birds, as also in a 

 similar condition of the carotid arteries. In the human brain the 

 hinder lobe of one of the hemispheres, more commonly the left, is 

 longer than the other, as may easily be seen in the cranium by the 

 depth of the corresponding fossaj. In the illustrious Bichat, the in- 

 equality of the two sides of the head amounted to deformitv. The 

 most striking instances of all are to be found in the halibut {Hippo- 

 glossug), fiounders {Pleuronectes), and other flat fishes, in which the 

 bones of the face, the brain and organs of sense are all moi'e or less 

 distorted, and the eyes especially are unsymmetrically placed. 



All organs wdiicli are thus unlike or unsymmetrical in the adult, 

 have been shown by embryologists, as by Steenstrup in the case of 

 the flat fishes, and also in the crabs, molluscs, etc., by other observers, 

 to be alike in the embryo, the deviations from true symmetry takin-T 

 place as development advances. We have seen a lobster nearly three 

 inches in length, in which the right and left anterior claws were still 

 symmetrical. 



The organs of the great cavities in adult vertebrates are almost uni- 

 formly unsymmetrical ; nevertheless in the embiyo the symmetry of 

 these parts, even of the liver, is complete. In some fislics this is true 

 of the liver of the adult, and in a few instances this is divided into a 



