COMPARATIVE ANATOMY 27 



Attempts have been made to trace homologies between the 

 basalia and radialia of sharks and the bones of the arm and the 

 hand of higher vertebrates. 



The pelvic girdle is much simpler than the pectoral and con- 

 sists merely of a bar of cartilage. 



The pelvic fin is also simpler than the pectoral. It consists 

 of: (a) two basalia, (b) many radialia (in males the posterior one 

 is much elongated to form the support for the clasper), and (c) 

 actinotrichia, as In the pectoral fin. 



Girdles and Limbs of Necturus 



Study a mounted skeleton. The pectoral girdle Is somewhat 

 primitive and consists mainly of cartilage. Extending dorsally 

 from the glenoid surface (the point of articulation of the arm) 

 Is a short scapula, the only part of the girdle composed of bone. 

 Attached to the distal end of the scapula Is a cartilaginous 

 suprascapula. The large cartilage extending medially from the 

 glenoid surface is the coracoid region; the one extending anteriorly 

 is the procoracoid. 



The forelimb is composed of humerus, articulating with the 

 girdle; ulna and radius, the bones of the forearm; carpus (wrist) 

 of several cartilages; metacarpus of four bones; and the phalanges 

 of the digits. The thumb is missing. Compare the specimen 

 with a chart to understand the relationship of the cartilages and 

 bones of wrist and hand. Also, compare Necturus with the 

 chart of the typical vertebrate limb. 



The pelvic girdle represents a transition stage between the 

 cartilaginous condition of the elasmobranchs and the bony girdle 

 of reptiles. It consists of a pelvic plate. The socket for articu- 

 lation of the leg is known as the acetabulum. Medial and ante- 

 rior to the acetabulum lies the pubic region; posterior to this is 

 the ischial regio7i; attaching the girdle to the vertebral column 

 is the ilium. Ilium and Ischium are partly bony. 



The hind limb Is like the forelimb. It Is composed oi femur, 

 articulating with the girdle at the acetabulum; tibia and fibula, 

 bones of the shank; tarsus of several cartilages; metatarsus of 

 four bones; and phalanges of the digits. The first toe Is 

 missing. 



