454 OSBURN 



remains, to be quite straight. Possibly the very great reduc- 

 tion in size of the limbs of the latter genus may be responsible 

 for this condition. 



Frequently the paddle is extended backward by the separa- 

 tion of the digits at their distal extremities so that the posterior 

 digits are thrown back at an angle to the others instead of being 

 parallel or nearly so. This is never found in the anterior border 

 of the paddle, but when present is always posterior. Many of 

 the Cetacea, such as Phoccena, Monodon, DelpJiinapteriis, Globio- 

 cephahis^ BalcBua, etc., exhibit this to a remarkable degree. In 

 the Mosasauria the angle at which the fifth digit is set is remark- 

 able, especially in the very short limbed Clidastes velox and in 

 Platecarpiis. Other forms may show this to a less extent and 

 the third and fourth digits may also be somewhat separated. 

 The Thalattosuchia show but little separation of the digits and 

 in Plesiosauria and Ichthyosauria the digits are remarkably close 

 together. Again it must be remarked that in Baptanodon, the 

 latest of the Ichthyosaurs, there is some separation of the digits. 



Another factor which sometimes enters to cause a backward 

 extension of the limb is hyperdactyly, or the addition of extra 

 digits on the posterior side of the limb. According to Baur 

 this addition of extra digits never occurs on the anterior border. 

 In the Cetacea very rarely a sixth digit, or part of one, is added. 

 In Ichthyosaurs it is quite common and as many as four extra 

 digits have been noted in the hand of /. longbnanus. Hyper- 

 dactyly does not appear in any of the other groups. 



The greater length of the digits toward the posterior border 

 as compared with the anterior ones causes a sloping off toward 

 the posterior border that gives much the same effect as the 

 curving of the digits, with which it is almost always coupled. 

 In almost all cases the first digit is noticeably shorter than 

 those following and in many cases the second and even the third 

 may be somewhat abbreviated so that a graduated slope is 

 formed from the first to the fourth. In the few cases where the 

 first digit is equal to or greater than those following, as in the 

 foot of Mixosaiiriis and the hand of Ichthyosaurus ingens or /. 

 conybeariy the digits are greatly curved. However, the hind 



