THE ENDOSKELETON 707 



tor foramen and ischio-pubic fenestra are united ; all three pelvic bones 

 unite to form the acetabulum, although the ilium and ischium may ex- 

 tend in such a manner as to exclude the pubis from taking part in the 

 formation of the fossa. Often an acetabular or cotyloid bone is formed 

 between the ilium and pubic bones and this may fuse with any of the 

 bones with which it comes in contact. 



The inter-pubic cartilage in marsupials and monotremes may or 

 may not persist throughout adult life. When it disappears and the bones 

 unite solidly, but do not definitely ankylose, such union is called a sym- 

 physis. 



In these non-placental mammals (the marsupials and monotremes 

 just mentioned) there are also marsupial bones which first form in car- 

 tilage, and then extend forward from each pubis in the ventral abdominal 

 wall. Their homology is unknown. 



THE FREE APPENDAGES 



In those animals, such as fishes living entirely in the water, the ap- 

 pendages are called ichthyopterygia. These are always paired fins. 

 When definite legs or arms are formed, such as in all classes of the 

 tetrapoda, such limbs and their modifications are known as chiropterygia. 

 It is commonly supposed that the latter have developed from the former, 

 but no one has yet been able to explain a method by which it came about. 

 All explanations, however, assume that certain parts of primitive fins 

 were retained and others likely modified, or, that certain parts were lost, 

 which were originally present, the remaining parts then becoming mod- 

 ified. The lower ganoids have a primitive form of fin but with increas- 

 ing complexity, there is a reduction of the basalia, either by entire dis- 

 appearance or by fusion. The remaining ones are then modified, so that 

 in elasmobranchs of the present time, we find the basalia usually number 

 three in the pectoral and two in the pelvic fins, being named from before, 

 backwards, as the pro, meso-, and meta-pterygium (Fig. 416)). The 

 middle one is absent in the hind limbs. The radiales are jointed trans- 

 versely, so as to give more flexibility. If these are arranged entirely 

 on one side of the basalia, they are called uniserial, but if they occur also 

 on the post-axial side, they are called biserial. The male elasmobranch 

 has the pelvic fin divided into two lobes, the medial being called the 

 clasper, or mixipterygium. 



The anterior portion of the pectoral fin may develop as a strong 

 defensive spine, sometimes connected with the poison gland. In eels the 

 pelvic fin is lacking. 



THE LIMBS 



The legs (chiropterygia) of all tetrapoda are essentially alike (Fig. 

 420). Each consists of several regions, comparable in detail with each 

 other. The proximal is the upper arm (brachium) or thigh (femur) con- 



