468 OSBURN 



the type of limb best suited for locomotion in the water, as wit- 

 ness the fins of fishes with their many-jointed rays giving them 

 an even flexibility. 



Concentration of all Parts Except the Digits. 



A little comparison will serve to convince any one that the 

 elongated type of propodial and epipodials with which we are 

 familiar in terrestrial forms is wholly unsuited to perfect loco- 

 motion in water. In all reptiles and mammals which have 

 become permanently aquatic the shortening up of this region of 

 the limb is so evident as to need little comment. The most 

 highly specialized forms in this regard are found among the 

 Ichthyosaurs in which the humerus may be so shortened that 

 its length scarcely exceeds its breadth (cf. SJiastasmiriis), while 

 the epipodials in nearly all cases are even shorter than wide. 

 The Thalattosuchia (cf. Geosmiriis) present the curious anomaly 

 of a very greatly shortened fore limb, while the hind limb is 

 little modified. Williston is probably right in thinking that this 

 hind limb of Geosaiinis was of little use in the water and was 

 probably carried close to the tail in swimming, after the manner 

 of newts. Among the Mosasaurs Clidastes and Plioplatecarpiis 

 seem to be the most highly specialized in this respect, while 

 among the Plesiosaurs, Cimoliosaiiriis, and in the Cetacea, 

 Globiocephalits, are among the most modified. 



Another factor in the concentration of parts is the shifting of 

 certain elements into new positions. Chief among these may 

 be mentioned the shifting of the so-called " pisiform " or its 

 homolog (cf. Baur ** On the Morphology and Origin of the Ich- 

 thyopterygia ") to a position alongside of the epipodials articu- 

 lating with the humerus or femur ; the shifting of the fifth digit 

 to articulate directly with the ulna as seen in some whales, per- 

 haps best in GlobioccpJiahis, and the Thalattosuchia ; and the 

 massing together of the bones in the carpal and tarsal region 

 against the ends of the epipodials, so that the intermedium is 

 sometimes interlocked between their ends. Concerning the 

 *' pisiform " it is necessary to state that there is grave doubt as 

 to the identification of this bone. Williston holds that the extra 



