PEDIGREES. 171 



backbone. In many living fishes, and a large 

 number of fossil forms, the "backbone" is repre- 

 sented only by this unsegmented gelatinous rod, 

 around which are arranged the A-shaped " neural 

 arches " for the spinal marrow, and the elements 

 for the support of ribs and protection of blood- 

 vessels. In many fossils we find these separate 

 elements preserved and arranged evenly around 

 a space. This indicates that the space was filled 

 by the very perishable gelatinous "notochord," 

 and tells us that the vertebral column retained 

 permanently the unsegmented and unhardened 

 condition such as we find in the embryos of 

 to-day. 



The modern type of shark made its first 

 definite appearance so far back in the world's 

 history as the period known as the Lias. We may 

 distinguish two groups of sharks, the one embrac- 

 ing the sharks and dog-fish which have an " anal 

 fin" (pp. 12, 61), the other certain dog-fish and 

 the rays in which the anal fin is wanting. If my 

 readers will forgive the introduction of apparently 

 long-winded names, he will find it useful to 

 remember that these two groups are known 

 respectively as the Asterospondyli and Tecto- 

 spondyli, in allusion to the characters of the 

 vertebrae. In the Tectospondyli (covered verte- 

 brse) the vertebrae are strengthened by con- 

 centric layers of hardened tissue : in the Astero- 

 spondyli the strengthening tissue is mainly 

 arranged in the form of lines radiating from a 

 common centre, hence the name Asterospondyli 

 (star-vertebrae). The sub-order Asterospondyli, 

 or sharks and dog-fishes with an anal fin, con- 



