POSTEEIOE EXTEEMITY OF VEBTEBEATA. 187 



the two pubic bones, and this is still more marked in the higher 

 orders. 



An independent adaptation, which is seen in various Mammals 

 (Iusectivora and Chiroptera), is the presence of a ligamentous con- 

 nection instead of the pubic symphysis ; this may be very wide in 

 the female (Erinaceus). 



When the posterior extremities are absent, the pelvic girdle also 

 undergoes atrophy. There are rudiments of it in the Cetacea. 



In the Monotremata and Marsupialia there are two bones in 

 front of the pubes ; these marsupial bones are directed forwards ; in 

 Thylacinus they are reduced to small rudiments in cartilage. 



Gegenbaur, C, Beitriige zur Kenntniss des Beckeus der Vogel. Jen. Zeitschr. 

 VI. — Hoffmann, C. K., Beitnige zur Kenutniss des Beckens der Amphibian 

 u. Reptilieu. Niederland. Arch. III. 



Posterior Extremity. 

 § 367. 



Wo find just the same arrangements in the hind-limb as we have 

 described as existing in the fore-limb. In Fishes the hind limb forms 

 the ventral fin. In the Selachii its skeleton has the same characters 

 as that of the thoracic fin ; the most striking difference is that the 

 rays are arranged in a simpler manner. The basale of the stem is 

 generally greatly elongated. The joints which succeed the basal 

 piece undergo a special metamorphosis in the male, where they are 

 converted into copulatory organs. 



The skeleton of the ventral fin in the Ganoidei may be derived 

 from this by supposing that there has been a peripheral reduction, 

 very similar to that which we saw in the skeleton of the thoracic 

 fin ; and the Teleostean fin can be derived from the Ganoid. This 

 is generally much simplified, both as regards the size and the 

 number of its separate pieces, in consequence of the feebler develop- 

 ment of the whole ventral fin. In both divisions the dermal skeleton 

 takes part in increasing the surface of the ventral fin, just as it has 

 been shown to do in the thoracic one. 



When we come to compare the hinder extremity of the higher 

 Vertebrata with the ventral fin of Fishes, we must again begin with 

 the archipterygium, which seems to be the lowest stage of this 

 extremity also. The segmentation of the extremity into successive 

 pieces is a repetition of the arrangement which we met with in the 

 skeleton of the arm. We distinguish the femur, tibia, and fibula ; 

 and lastly, in the foot, a tarsus, metatarsus, and phalanges. The 

 four inner toes, and the parts that carry them, may be again 

 regarded as joints of the rays which are given off from a row of 

 bones extending from the femur, through the fibula, to the 

 outermost toe. The tarsus is made up of ten pieces, three of which 



