ESPECIALLY MYCTOPHOIDS 233 
stock. For example Pattersonichthys delicatus differs from Sardinioides minimus in 
the following features : 
1. A small broad ascending process with a short alveolar arm on the pre- 
maxilla. 
Long, stout maxilla which is extensively toothed, forming two-thirds of 
the gape. 
Two very large supramaxillae. 
Unexpanded suboperculum. 
Sub-thoracic pelvic fins. 
Cycloid scales which do not extend on to any of the skull bones. 
Fulcral scales reduced in extent. 
SOE a 
ing 
Apart from these points the two genera are practically identical in all respects. 
Thus two groups (Ctenothrissiformes and Myctophiformes) which are ordinally 
separated have basal members which converge strongly, although the ctenothrissi- 
form and myctophiform lineages are evident in the convergent stocks. Patterson 
(1967a : 104) has considered a similar basal convergence of the trachichthyid and 
holocentrid berycoids at the base of the Cenomanian. 
Although it has now been indicated that the basal myctophiforms and cteno- 
thrissiforms are on equivalent levels of structural organization, the evolutionary 
potential within either group would appear to be different. The ctenothrissiforms 
were endowed with the ability to produce a higher level of organization, the Acantho- 
pterygii (sensu Greenwood, et al., 1966). The evolutionary potential in the mycto- 
phiform lineage would appear to have been limited solely to producing diversified 
members of the same structural level of organization, although certain of the 
descendants are extremely specialized for particular habitats. This viewpoint, 
however, excludes the possibility of the basal myctophiforms having given rise to the 
Paracanthopterygii (the parallel spiny-finned radiation) as Greenwood, et. al., 
(1966) supposed. If the myctophiforms did give rise to the Paracanthopterygii then 
the evolutionary potential of the myctophiforms and ctenothrissiforms was similar. 
IV. TELEOST EVOLUTION WITHIN THE CRETACEOUS 
Before the evolution of the teleosts in the Cretaceous can be considered it is 
necessary to give some discussion of the evolution of the teleosts as a whole. The 
first question to be answered is whether or not the teleosts represent a monophyletic 
group or a polyphyletic one. Patterson (1967c : 95) has given this question some 
thought but indicates that one must first settle upon the definition to be attached 
to the term polyphyletic. Recently most authors have considered that the teleosts 
had their origin within the halecostomes. Thus monophyly occurs at the level 
of the division Halecostomi. If, however, within the halecostomes it is found that 
more than one family had given rise to the teleosts, then polyphyly is present at 
the familial level. This point is discussed in some detail by Patterson (1967c), 
who is of the opinion that no grounds have so far been put forward which detract 
