VERTEBRA AND RIBS 



35 



DIPNOI. — -Although higher than the Teleosts in several respects, 

 the lung fishes have a very primitive vertebral column, standing in 

 this respect on a level with the HolocephaH. Distinct centra are 

 lacking, except at the anterior end of the column where two or three 

 neurals and haemal s may unite in a ring around the notochord. Else- 

 where the neural arches rest on the partly calcified notochordal 

 sheath (fig. 38), meet above the longitudinal ligament, and are 

 continued dorsally by long spinous processes, which may be jointed 



Fig. 38. — -Vertebras of Lepidosiren (Bischoff, '40). A, just behind head; B, middle of 

 tail; h, haemal arches; n, neural arches; nc, notochord; r, ribs. 



•and usually are the supports of the dorsal fin. The caudal haemal 

 arches are much like the neural and usually have spinous processes. 

 Intercentra occur only in the tail, caudal to the neural arches (appar- 

 ently cranial elements) these being rudimentary in Protopterus. The 

 centra are larger in the posterior caudal region of Ceratodus, and near 

 the tip of the tail (fig. 39) is a series of centra-like bodies with ossi- 



FiG. 39. — End of vertebral column of Ceratodus (Giinther, '71). c, end of notochord. 



fied neural and haemal spines. The anterior of these centra contain 

 notochordal tissue, but farther forwards this is replaced by cartilage. 

 The vertebrae are cartilage, but may have a strong external layer of 

 bone. There are haemal ribs in the trunk which are continuous with 

 the haemal cartilages on the notochord, the anterior of these being 

 peculiar in being separate from the column and attached to the 

 cranium (the so-called head rib, fig. 122). 



TETRAPODA.— In Tetrapoda the notochord is relatively 

 smaller than in fishes, being best developed in Amphibia (fig. 43), 

 where both elasticae are present, although the tissue between them is 



