In view of this historical uncertainty of the relationships between the
Nitosauridae, the Eothyrididae and the Caseidae, it is well to consider
how the maxillary fragments described above differ from and resemble
representatives of each of these three families as reported in the
literature.

_Delorhynchus_ resembles _Colobomycter_ in size. The mean
extra-maxillary length of the undamaged teeth of the three fragments is
2.5 mm., equal to that reported by Vaughn (1958:985) for teeth about
midway in the postcanine series of _Colobomycter_. None of the teeth of
_Delorhynchus_ extends beyond the maxillary rim as far as does the
canine of _Colobomycter_ (3.5 mm.).

The teeth in both genera are conical and sharply pointed. The naris in
each is enlarged, and the lacrimal is excluded from the narial margin in
each (by inference in _Delorhynchus_.)

The differences between the maxillae of _Colobomycter_ and
_Delorhynchus_ are most striking in the lack of canines in the latter
and the correlated absence of modifications of the maxillary for support
of canines. Additionally, _Delorhynchus_ bears an infraorbital canal in
contrast to the groove in similar position in _Colobomycter_. The
recurvature of the four teeth present in the fragments of _Delorhynchus_
differs from that in the teeth of _Colobomycter_ in which only the
canine and precanine are recurved. Vaughn implies that anterior and
posterior cutting edges extend the length of the teeth in
_Colobomycter_; these are restricted to the distal third of the teeth in
_Delorhynchus_. The external surfaces of the maxillae of _Delorhynchus_
are pitted and ridged; Vaughn was unable to discern sculpturing of the
corresponding surfaces in _Colobomycter_.

_Delorhynchus_ resembles the nitosaurids in size, the shape and
sharpness of the teeth, their recurvature and the slight enlargement of
their bases, the exclusion of the lacrimal bone from the narial margin
(in _Mycterosaurus_) and the apparent lack of a special canine pair of
teeth. Resemblances to the caseids are to be noted in the enlargement of
the naris (4.5 mm. in height as opposed to 1.7 mm. in _Colobomycter_),
lack of development of canines, presence of an infraorbital canal (in
_Cotylorhynchus_) and absence of many replacement gaps in the marginal
row of teeth.

The absence of caniniform enlargement and the extension of the maxilla
dorsad of the naris exclude _Delorhynchus_ from the Eothyrididae
(Ophiacodontia) but ar