Aug., 1847.] 243 



were recognized, but they were not sufficiently distinct to be re- 

 ferred to any class. 



Thuse vegetable impressions I would refer to the Henopteris 

 or Glossopteris, and are new. 



In the Geological Keport of the State, this formation is placed 

 below the Tertiary." 



The Chairman read a letter from Dr. Jeffries Wy man, dated 

 Boston, Aug. 5, 1847, enclosing sketches of crania of a male 

 and female supposed new species of Orang, inhabiting 

 Western Africa, brought to this country by Dr. Thomas S. 

 Savage. Information on the subject was requested from the 

 Society. 



Dr. Morton read a portion of a memoir designed for pub- 

 lication in the Journal, " On the position of the Ear in the 

 statuary and paintings of the civilized nations of antiquity ;" 

 which was referred to a Committee consisting of Mr. Grliddon, 

 Dr. Pickering and Dr. Wilson. 



Dr. Leidy presented an instance of a curious optical illusion 

 arising from a peculiarity in the structure of the intervertebral 

 substance of man. He stated the external third of each disk to be 

 composed of concentric layers of non elastic or white fibrous tis- 

 sue, the fibres of which are oblique, those of the alternate layers 

 crossing each other. This arrangement, when viewed from either 

 side, gives rise to the appearance of alternating layers of while 

 fibrous tissue, indicated by its shiniug whiteness and opacity, aod 

 of cartilage, indicated by its semi-transparency and duluess, and 

 this is actually stated to be the arrangement in anatomical works 

 generally, and even in the late and very excellent Physiological 

 Anatomy of Messrs. Todd and Bowman. 



The illusion is well marked : if a disk be held side-ways in the 

 hand, and pins be stuck in what seem to be the alternating layers 

 of cartilage, and then viewed from the other side, the pins will 

 actually appear to have changed their position from the cartilage 

 to the fibrous layers. 



Dr. L. explained the illusion by supposing it to be produced 

 in the one case from the rays of light reflected from the numerous 

 points of the fibres, when the eye is opposed to them, giving rise 

 to a divided, and consequently white, opaque surface, while in the 

 other direction, the rays of light being partly transmitted by the 

 extremities of the fibres, give rise to the characteristic dulness and 

 semitransparency of cartilage. 



