234 UPPER CRETACEOUS TELEOSTS 
from the idea that the teleosts are monophyletic at the level of the family Pholido- 
phoridae. 
The division Halecostomi represents a transition grade between the basal holo- 
steans and the teleosts and was first used by Woodward (1932). The central stock 
of the halecostomes would appear to be the family Pholidophoridae, and Gardiner 
(1960) has convincingly shown that this family can be derived directly from the early 
Parasemionotiformes. The family Pholidophoridae is normally grouped with five 
other families in the order Pholidophoriformes (see Griffith and Patterson, 1963, for 
consideration of these families). The Pholidophoridae appears to be closest in 
structural organization to the teleosts whereas the other families are more distantly 
removed. 
The Pholidophoridae sensu stricto have recently been extensively examined by 
Nybelin (1966) who agrees with Gardiner (1960) that they were derived from a 
parasemionotid-like ancestor. In confirmation Griffith and Patterson (1963 : 37) 
stated that the parasemionotids are almost ideal intermediates between the chon- 
drosteans and the halecostomes. 
The division Halecostomi also includes the order Leptolepiformes, and this order, 
together with the Pholidophoriformes, presents a collection of families which 
exhibit a mosaic of halecostome and teleostean characters (mosaic evolution is used 
in the sense that de Beer, 1954, and Schaeffer, 1965, indicated, viz. lineages of com- 
mon ancestry exhibiting various combinations of primitive, intermediate and ad- 
vanced characters). Schaeffer (1965 : 322), in reference to the origin of the teleosts, 
indicated that every organism is a mosaic whether or not it is associated with a 
transition between grades or with an adaptive radiation within a grade. The latter 
point has been amply illustrated in the foregoing discussions where radiations 
within the Salmoniformes have been considered. It is hoped that the former point 
will become apparent throughout this section. 
This mosaic nature of the halecostomes makes it difficult to define a ‘ teleost ’. 
Patterson (1967c) has argued the ‘ pros and cons ’ of the dilemma and whether or not 
it is advisable to recognize the Teleostei as a taxonomic group. Lehman (1966) and 
Berg (1940) for example do not recognize a division Teleostei, but Patterson 
(1967c : 95) finds that a category Teleostei is necessary and that the base of this 
group is within the leptolepids. 
The teleost characters shown by the halecostomes are related to changes in 
respiratory, feeding and locomotor mechanisms and include : 
1. Loss of enameloid tissue (ganoine) from scales and dermal bones. 
2. Thinning of scales and loss of the peg and socket joint. 
3. Ossification of the vertebrae as single units. 
4. Development of numerous intermuscular bones. 
5. Compact caudal skeleton with two hypurals to the lower caudal lobe 
supported on ural vertebra one. 
These features all indicate the increased flexibility of the body and compensate in 
part for the increased mechanical stresses built up by the musculature (Nursall, 1956). 
One primitive character of the halecostomes is the presence of enamel as a thin layer 
