256 THE DOCTRINE OF DESCENT. 



Our knowledge of the genealogy of the fishes may 

 be laid down in the following diagram: — 



Dipnoi. 



Teleostd. 



Ganoids. 



I 



Elasniobranchii. 



Marsipobranchii. 



The Marsipobranchii (Cyclostomi), it is true, exhibit 

 important peculiarities, such as deficiency of limbs, en- 

 tire absence of bony plates or scales on the integument; 

 but the brain, heart, and vertebral column (which, al- 

 though persistently cartilaginous, is far superior to that 

 of the Amphioxus), show their direct connection with the 

 fishes. Fossil remains of these animals, universally known 

 in the genus lamprey (Petromyzon), are not forthcom- 

 ing, and, at the most, only their horny teeth could have 

 been preserved. 



After these manifest gaps in our knowledge, the suc- 

 ceeding orders of fishes present themselves in a connec- 

 tion all the more conspicuous. The starting-point is 

 formed by the Elasmobranchii, to which belong the true 

 chimeras, sharks, and rays. Brain and gills testify their 

 kindred with the Marsipobranchii. In the construction 

 of the cranium, facial bones, pectoral and pelvic arches, 

 and the anterior extremities, heart and intestine, they 

 exhibit forms to which, as Gegenbaur has shown in his 



