188 UPPER CRETACEOUS TELEOSTS 
the roofed post-temporal fossa ; the small, discrete laterally facing dilatator fossa 
above the hyomandibular facet ; toothed vomer ; simple nature of the premaxilla ; 
long narrow maxilla which rests on a shelf on the lateral face of the mandible when 
the mouth is closed ; vertical jaw suspension ; absence of an interoperculum ; 
presence of numerous intermuscular bones, and abdominal pelvic fins. 
The rostral region of Prionolepis is somewhat lengthened, much as in Cimolichthys, 
but the body is little elongated. One of the most distinctive features of Prionolepis 
is the mesethmoid arrangement (Text-fig. 83). The anterior rod-like portion with 
which the premaxillae associate and the two large postero-laterally arranged 
flattened wings are different to the condition of this region in either Cimolichthys 
levesiensis or the dercetids. However in Cimolichthys nepaeolica from the Niobrara 
Chalk of Kansas, the mesethmoid arrangement approaches the condition in Priono- 
lepis. As with Cimolichthys levesiensis (p. 40, Text-fig. 16), Cimolichthys nepaeolica 
(Hay, 1903, fig. 69) has the same large backwardly divergent laminae, but anteriorly 
there is a forwardly projecting prominence closely resembling the straight rod-like 
piece in Prionolepis cataphractus (p. 182, Text-fig. 83). This mesethmoid arrange- 
ment appears to be distinct from all other teleost groups and provides good evidence 
for the relationship between Cimolichthys and Prionolepis. 
There are no expanded transverse processes on the precaudal centra, in this 
respect differing markedly from the Dercetidae. Cimolichthys, however, also has 
no such expanded processes although it is obviously closely related to the dercetids. 
The deepened, shield-like flank scutes in Prionolepis are of interest (Text-fig. 85) 
since they show signs of being feebly ctenoid. However the scales look more like 
the pectinate scales of Lepidotes rather than true ctenoid scales. The deepened 
flank scutes recall those of Aspidorhynchus and Belonostomus (Gardiner, 1960), and 
among recent fish similar scutes are exhibited in the Gasterosteiformes. It is of 
interest to note that in the gasterosteoids the flank scutes are supported on large 
transverse processes. 
The systematic position of the Cimolichthyoidei is not at all clear. Pictet (1850) 
originally used the family Dercetidae (Hoplopleuridae) to include the genera 
Dercetis, Leptotrachelus, Pelargorhynchus, Saurorhamphus and Eurypholis, being 
those fishes devoid of normal squamation but possessing isolated scute rows. 
Woodward (1go1 : 171) used Pictet’s family Dercetidae but removed Saurorhamphus 
and Eurypholis to the Enchodontidae. Since 1gor one further genus has been 
added by Arambourg (1944 : 281), Rhynchodercetis. 
The dercetids in the past have been associated with one or another of two major 
groups. Woodward (1gor) related the dercetids to the halosaurs and notacanths. 
Jordan (1923 : 128) more or less concurred and grouped the Dercetidae (now includ- 
ing Prionolepis and Leptecodon) in the order Heteromi stating that they were close to 
the notacanthids. 
The more predominant line of thought, however, has been that the dercetids are 
more nearly related to the myctophiform assemblage. Regan (1911 : 120) stated 
that the orbital, postorbital and post-temporal parts of the Dercetidae resembled 
Evermannella, whilst the ethmoid region and jaws resembled Alepisaurus. 
