VERTEBRAL COLUMN 



233 



sc 



No actinotrichia have been observed in young, or adult, 

 Dipnoi (pp. 212, 273). 



The vertebral column has a persistent unconstricted notochord. 

 Although, judging from living forms (Fig. 204, A), the elastica 

 .externa is ruptured, and the mesoblastic cells invade the thick 

 fibrous notochordal sheath to some extent, yet complete vertebral 

 bodies are never formed so as to constrict the notochord in the 

 typical manner. But in Ceratodus the notochord is pushed inwards 

 alternately, above and below, by the 

 .cartilaginous masses, which grow from 

 the basidorsals and basiventrals (Fig. 

 204, B). The extreme tip may be 

 entirely surrounded by cartilage. 



There is, then, a thick fibrous sheath 

 in which are partially embedded cartila- 

 ginous basidorsals and basiventrals. Small 

 .and irregular intercalaries (interdorsals 

 .and interventrals) are occasionally present. 

 The basidorsals extend upwards as ossified 

 neural arches meeting above the nerve- 

 -cord ; along the top of these arches runs 

 & longitudinal ligament, above which 

 again are a series of neural spines. The 

 latter may be fused to the arches in 

 the hinder region. Similarly the caudal 

 region is provided with haemal spines 

 and arches, continued in front into 

 basal cartilages and pleural or ventral 

 ribs (Fig. 204, C) lying just outside 

 the peritoneum. There are no true 

 ribs. 



All living Dipnoi are provided with 

 a diphycercal tail, and continuous dorsal 

 and ventral median fins (Fig. 205). 

 Dipterus, however, has two separate dorsal 

 and one anal fin, and a heterocercal tail 

 {Fig. 226). In Phaneropleuron with a 

 separate anal only, and Scaumenacia with 

 an anal and two dorsal fins, we have 



intermediate conditions (Fig. 227). Dollo [120] has very plausibly 

 argued that these Dipnoi represent a degenerating series leading 

 to an extreme form like Lepidosiren, in which the body becomes 

 more elongated and eel-like, the tail becomes straightened, and the 

 median fins fuse together. It must be remembered, however, that 

 .the Carboniferous genus Uronemus already has continuous median 



FIG. 203. 

 Diagram of a section through 



scale. (From 



