Chap, ix.] FINS OF FISHES. 359 



by the hind limb, though it can be regained under the 

 stress of necessity, or by education ; the saddle-shaped 

 form of the articular surface of man's trapezium gives 

 the mechanical reason for the power of apposition of 

 the thumb, which he possesses in so marked a degree. 



In the tendons of the digits extra bones (sesa- 

 moids) are not unfrequently developed, and their 

 presence is no doubt to be explained by a refer- 

 ence to the primitively multiradiate condition (see 

 page 361) of the vertebrate limb; of such bones the 

 most constant is the patella (knee-cap), which is 

 found in all Mammals save a few Marsupials ; 

 another, which is very frequently found in the carpus, 

 is the so-called pisiform (or pea-shaped bone of the 

 human hand). The sesamoids are, as will be imme- 

 diately explained, most commonly developed in asso- 

 ciation with the digits ; thus, in the dog they are 

 found on each metacarpal ; in the fossorial armadillos 

 there is a large sesamoid on the palmar side of the 

 metacarpus ; two large palmar sesamoids are found in 

 Ornithorhynchus ; while in the just-mentioned Mono- 

 treme, as to a less extent in Echidna also, there is a 

 large sesamoid in the tarsus which supports the spur 

 of the foot, that has so remarkable a likeness to what 

 is found in the fowls and some other birds. 



The paired fins of Fishes are, at first sight, 

 difficult to bring into alliance with the pentadactyle 

 limb of the higher Yertebrata. If we take the dog- 

 fish as a type, we find that the pectoral are larger 

 than the pelvic fins, and more complicated in cha- 

 racter. We will commence, therefore, with an account 

 of the latter. They lie horizontally, and approach 

 one another at the ventral median line. A long basal 

 bar (Fig. 154; A, bp) is articulated to a process of the 

 ilium, and bears on its outer side a series of rays, which 

 are each divided into a larger proximal or basal and a 

 smaller distal piece, almost parallel to one another; 



