256 THE DOCTRINE OF DESCENT. 
Our knowledge of the genealogy of the fishes may be 
laid down in the following diagram :— 
Dipnoi. 
Teleostei, 
x 
Ganoids. 
Elasmobranchii. - 
Marsipobranchii. 
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 (which, 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 
