256 THE DOCTRINE OF DESCENT. 



Our knowledge of the genealogy of the fishes may be 

 laid down in the following diagram : — ■ 



Dipnoi. 



TclcosteL 



Ganoids. 



EListnobranchii. 

 IManipobranchii. 



The Marsipobranchii (Cyclostomi), it is true, exhibit 

 important peculiarities, such as deficiency of limbs, entire 

 absence of bony plates or scales on the integument ; but 

 the brain, heart, and vertebral column (w^hich, although 

 persistently cartilaginous, is far superior to that of the 

 Amphioxus), show their direct coherence with the fishes. 

 Fossil remains of these animals, universally known in 

 the genus lamprey (Petromyzon), are not forthcoming, 

 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 



