408 me. k. av. hoolet on xhe [June 1 91 3, 



In Nyclosaurus 1 the pteroid is greatly developed, and the proximal 

 end on the lateral border has a wing-like projection at right angles 

 to the shaft, which is not seen in Omithodesmus. 



As the question of whether the pteroid is the first digit or a 

 separate ossification is still open, it will be as well to state what 

 light is cast on the point by the study of OrnitJwdesmus. I 

 have before stated that the proximal end was overlapping by 

 15 mm. the distal end of the radius. On its interior concave face 

 there are no signs of muscle-striae, while there are on the exterior 

 surface lines converging proximally and apparently continued 

 round the border of the bone, the whole appearance suggesting 

 that the bone was affixed by muscular attachment. Moreover, the 

 extreme proximal exterior surface is bevelled off, in such a manner 

 that the edge is quite sharp ; whereas, if articulating with the 

 lateral carpal, it would at least be obtuse. The peculiar, large, 

 oblong foramen on the inner surface is perhaps not pneumatic, 

 but rather incomplete ossification for the adhesion of the investing 

 tissue. Dr. Plieninger, in common with the majority of authors, 

 believes the pteroid to be a turned-back thumb, as suggested by 

 Hermann von Meyer. I agree with Prof. Williston {he. supra cit.), 

 that it is ' an entirely distinct ossification ' : because, if we concede 

 that it is the thumb thrown back, we must explain how such a 

 modification was accomplished. The thumb would assuredly be the 

 first digit that would be used in clinging to rocks or boughs for 

 support, and thus would have no incentive to reflex, as it would 

 be, on account of the wing, the nearest and best digit to set 

 in action for the grasp. We must suppose that both the ' thrown- 

 back thumb ' and the wing-finger were included in the patagium, 

 leaving the intermediate digits free; and, if the former were 

 reflexed, why not the latter? or, if the' wing-finger be the fourth 

 finger, as it would be if the pteroid is not the thumb, why is 

 not the fifth finger found reflexed, as the pull would be as great 

 on the one as on the other ? According to this theory, the so- 

 called ' pteroid ' described an arc, until its present position was 

 attained. There is nothing to prove that the wing-membrane was 

 anything more than a fringe (if that) down the preaxial border 

 of the arm, and this would not provide the powerful stimulus for 

 so extraordinary a change : the stress would be so insignificant, that 

 it cannot be conceived how it would be necessary as a stay, or as a 

 means of stretching the narrow frill of the wing. Xor, in the 

 position in which it is always found, nearly parallel to the ante- 

 brachium, could it enlarge or stiffen the spread of the wing in 

 flight. What, then, are the causes which by their action produced 

 such an effect ? We have good reason for looking upon the 

 ' pteroid ' as an ossified extensor-tendon : for the enormous dis- 

 rupting strain on this sinew by the weight of the lengthening wing- 

 digit, when cleaving the air in flight, would set up an irritation 

 within the tendon itself, which would cause ossification to take 

 place. Thus, naturally, the end of the ossification farthest away 



1 S. W. Williston, Geol. Mag. dec. 5, toI. i (1904) p. GO. 



