314 Evolution and Distribution of Fishes 



origin and ancestry, in view of what has been said regarding 

 these genera above. 



But the last and lingering remnant genus of ancient 

 Crossopterygians, viz. Macropoma is as yet a puzzle. Oc- 

 curring in Cretaceous rocks of Turonian and Senonian age, 

 the animal as described by Mantell, Dixon, Huxley and 

 others, seems to be associated only with marine strata, and 

 their related organisms. Unlike many fishes that have be- 

 come marine however, the tenacious retention of the ossified 

 air-bladder is noteworthy. 



In the oldest description by Mantell he states that 

 {Amia) Macropoma lewesiensis belonged to the "Lower 

 Chalk" of Sussex, as does also Dixon. Mantell however, 

 indicates (pp. 139-142) a great variety of beds, and vary- 

 ing organisms in these, when he says that the "Lower 

 Chalk" is regularly stratified, the lines of separation being 

 composed of a softer chalk that in some places contains 

 so great a proportion of argilla, as to assume the appear- 

 ance of marl. The latter also occurs in transverse and 

 vertical veins, in which the remains of fishes are more 

 frequent than in the more solid strata." And again he says 

 that eight or ten layers of chalk may be separated by as 

 many seams of chalk marl. 



In the elaborate account also of "The Lower and Mid- 

 dle Chalk of England" {2^3) Jukes-Browne has recorded 

 (p. 44) Macropoma from the Lower Chalk of the Kentish 

 coast, and there associated with a dozen other marine fishes. 

 Again speaking of the Middle Chalk he says (p. 369) : 

 "Remains of fish are abundant in some localities, especially 

 in the counties of Kent and Sussex. Many of the species 

 are the same as those occurring in the Lower Chalk, such 

 as Ptychodus deciirrens, P. mammillaris, Macropoma man- 

 telli^ and Beryx [Ctenothrissa) radians." 



The description and beautiful figure given by Fritsch 

 {161) also leaves one in doubt as to the environal strati- 

 graphy. One wonders however as to the meaning of the 

 above-recorded fishes being "in transverse and vertical 

 veins" also as to the exact fossils found in each chalk layer, 

 and in each alternating marl layer, as well as the chemical 

 composition of each. So until an extremely careful and ex- 



