ever, inAcipenserthey are not as dominantly elon-
gate and are thin. Similarly, Husoand all polyodon-
tids (Grande & Bemis 1991) possess paired first ven-
tral bones only modestly longer than other bones of
the series (see Character30).
orbit and do not open dorsally. Extant polyodontids
possess open orbits without boundaries other than
the bare olfactory bulbs (Figure 20a). They do pos-
sess a broad channel dorsomedial to the olfactory
bulb, but this is open for their protractor hyoman-
dibularis muscle and is not comparable to the orbi-
tal notch.
Character 43. Basitrabecular cartilages present -
Scaphirhynchini
Character 45. Posttemporal processes present -Sca-
Basitrabecular cartilages are present in all scaphi- phirhynchini
rhynchines lateral to the small basitrabecular pro-
cesses (Figure 15d.,e). The basitrabecular cartilage Posttemporal shelves are cartilaginous processes of
ofScaphirhynchusis large and curved (Figure 15d) the neurocranium that interlock with the posttem-
along the postnasal wall (Findeis 1993). It is much poral. In scaphirhynchines, they flare posterome-
smaller in Pseudoscaphirhynchus, being barely dially as thick processes with short distal tips that
larger than the diminutive basitrabecular process it- angle posteriorly. They possess medially angled an-
self (Figure 15d). InScaphirhynchusbasitrabecular teroventral laces that converge to the base of the
cartilages develop as outgrowths of the basitrabec- neurocranium. The dorsal surface is rounded and
ular processes that separate from the neurocranium indented from the surface of the neurocranium. Be-
in small juveniles. Sewertzoff (1928) identified ba- cause of their cylindrical shape, I refer to them as
sitrabecular cartilages as outgrowths of the palato- posttemporal processes (Findeis 1993).
quadrate and homologized them as pharyngoman- In all species ofAcipenserandHusoexamined,
dibulars, but they develop distinct from the man- the posttemporal shelves have flat lateral faces that
dibular arch. converge without curvature to the base of the neu-
All species of Acipenser examined, Huso, and rocranium. The dorsal edges are also flat, extending
Psephurus possess large basitrabecular processes smoothly from the dorsal surface of the neurocrani-
(Figure 15a, b, c), but no independent, accessory um and the posterolateral tips are short processes
cartilage. that extend linearly from the shelves. Polyodontids
possess posttemporal shelves similar toAcipenser
andHuso,but they are broader, flatter, and not as
Character 44. Orbitalnotchpresent-Scaphirhyn- elongate.
chiniThe orbital notch indents into the neurocranium
from the dorsal edge of the orbit (Findeis 1993). It
contains passage of the profundal ramus that scat-
ters onto the dorsal surface of the neurocranium
and the superficial ophthalmic ramus of the antero-
dorsal lateral line nerve. The orbital notch lies be-
neath the supraorbital canal and an ampullary field
anterodorsal to the orbit.
The dorsal orbit ofAcipenserandHusois semi-
circular, without indentation.Husoand all species
ofAcipenserexamined possess shallow anterodor-
sal depressions in the postnasal wall homologous to
the orbital notch, but they are restricted within theCharacter 46. Ventral process of theantorbitalis
elongate-ScaphirhynchiniThe antorbital possesses elongate ventral processes
along the postnasal wall in scaphirhynchines (see
Character 4). In Scaphirhynchus,it is a straight,
rectangular process contacting the postrostral bone
(Figure 4e; see Character 61). InPseudoscaphirhyn-
chus,the ventral process tapers with an anteriorly
curving point (Figure 4d). The elongate ventral pro-
cess correlates with the expanded rostrum (see
Character 39) to cover the postnasal wall.
The antorbital ofHusopossesses a short wedge