92 E, T. NEWTON ON FISHES' TAILS. 



JEchmodus, Semionotus, and Pholidophorus, from the Lias, Lepi- 

 dotus, which is found from the Lias to the Wealden, and Histio- 

 notus, from the Purbeck, all have tails approaching that of Lepi- 

 dosteus, and agreeing with stage 5. The Dapedius, from the Lias, 

 makes even a nearer approach to homocercality stage 5 or 7 ; but, 

 according to Agassiz' restoration,* it is in the Pachycormns from the 

 Lias, the Sauropsis from the Oolite, and the Leptolepis, which is 

 found from the Lias to the Purbeck, that we see the highest de- 

 velopment of the Ganoid tail, for externally they are just as much 

 homocercal as any Teleostean fish. We have no evidence to show 

 that this is really identical with the Teleostean homocercality, and I 

 think future research will prove that the Ganoid homocercality 

 differs from that of the Teleostean. At present they must be re- 

 garded as stage 9 or 11 of the flounders' development. 



3. Crossopterigidce. There is much variation in the tails of these 

 fishes, even among those which are found in the Devonian Rocks. 

 Thus : — The genera Glyptolcemus and Glyptopomus having the tail 

 straight and equal above and below, seem to resemble the earliest 

 condition of the embryo, stage 1 . 



Holoptychius and Gyroptychius may be taken to represent stage 2, 

 as they have a slight upward tendency. 



Glyptolepis has a greater development of the lower caudal lobe, 

 and represents the stage 3. 



Diplopterus has a distinct heterocercal tail, with a well-developed 

 lower lole, and would represent stage 4 or 5. 



Tristichopterus has a most remarkable tail. Its upper and lower 

 lobes are nearly equal, but the end of the body extends upwards 

 and backwards through this, so as to form another smaller lobe 

 within the other two. This tail is heterocercal, and would repre- 

 sent stage 6 or 7. 



In the Coal Measures the Crossopterigidas are found represented 

 by Holoptychius and Megulichthys, the former resembling stage 2, 

 while Megalichihys is most like stage 3. 



The Ccelacanthini, a peculiar group of the Crossopterigida?, 

 closely allied to the Dipnoi, are represented in the Coal Measure by 

 Ccelacanthus, and in the Mesozoic Rocks by Undina, Bolophagus, 

 arid Macropoma, in all of which the notochord was unossified and the 

 tail diphycercal, stage 1. No Crossopterigian is known in Tertiary 

 strata, but at the present day it is represented by Polyptems 



* " Poissons Fossiles," vol. L pis. A to G. 1833-43. 



