2004 
well developed; postorbital processes usually 
absent; petrosal usually exposed on lateral 
surface of braincase through fenestra be- 
tween squamosal and parietal bones; maxil- 
lary fenestrae absent; posterolateral palatal 
foramina small, not extending lingual to M4 
protocones; maxillary and alisphenoid not in 
contact on floor of orbit; extracranial course 
of mandibular nerve (V°*) enclosed by an- 
teromedial strut of alisphenoid bulla (second- 
ary foramen ovale present); fenestra cochleae 
exposed laterally (not enclosed in a bony si- 
nus); I2-I5 with approximately symmetrical 
rhomboidal crowns increasing in_ breadth 
from front to back; P2 and P3 subequal in 
height; molar dentition highly carnassialized; 
centrocrista strongly V-shaped, its apex high 
above trigon basin; lower canine (cl) pro- 
cumbent and premolariform (always with a 
flattened, bladelike apex and usually with a 
small posterior accessory cusp); p2 taller 
than p3; dp3 fully molariform; m3 hypoconid 
labially salient; entoconid large and well de- 
veloped on ml-—m3. 
COMPARISONS WITH MARMOSA: Species of 
Marmosops have long been confused with 
Marmosa, which they superficially resemble 
in size and external appearance, but these 
taxa are only distantly related (Jansa and 
Voss, 2000; Voss and Jansa, 2003) and can 
readily be distinguished by qualitative integ- 
umental and craniodental characters (table 2). 
In the field, two external characters are useful 
for generic identification. Whereas the manus 
of Marmosops is mesaxonic, with digit (d) 
III distinctly longer than the adjacent digits 
(dII and dIV; fig. 3A), the manus of Mar- 
mosa is paraxonic, with dIII and dIV sube- 
qual in length (fig. 3B). Easily determined 
from live specimens (or properly fluid-pre- 
served material) with elastic phalangeal 
joints that can be manipulated to align and 
straighten adjacent digits for relative length 
comparisons, this character is often hard to 
score from dried skins with hard, bent, twist- 
ed, or otherwise distorted fingers (see Voss 
and Jansa, 2003: character 10). A second dis- 
tinguishing external trait is the morphology 
of the caudal pelage, of which the central 
hair in each caudal-scale triplet is petiolate 
(narrower basally than at mid-length), thick- 
er, and usually more darkly pigmented than 
the lateral hairs in Marmosops; by contrast, 
VOSS ET AL.: BOLIVIAN MARMOSOPS 7 
the three hairs of each caudal-scale triplet are 
subequal in thickness and similarly shaped 
and pigmented in Marmosa (Voss and Jansa, 
2003: character 27). 
In dorsal cranial comparisons, these taxa 
are distinguished by the usual absence of dis- 
tinct postorbital processes of the frontal bones 
in Marmosops (fig. 4B) versus the usual pres- 
ence of distinct postorbital processes in adults 
of most species of Marmosa (fig. 4A). In lat- 
eral cranial view, the petrosal capsule that en- 
closes the paraflocculus and semicircular ca- 
nals (= pars canalicularis or pars mastoideus) 
is usually exposed through a fenestra between 
the squamosal and parietal bones in Marmo- 
sops (fig. 5A), but there is no such lateral pe- 
trosal exposure in Marmosa (fig. 5B). In ven- 
tral cranial view, Marmosops is distinguished 
by the invariant presence of a secondary fo- 
ramen ovale (fig. 6B), which is just as con- 
sistently absent in Marmosa (fig. 6A). 
Marmosops and Marmosa are similar in 
most dental traits, but the morphology of the 
deciduous lower third premolar (dp3) ap- 
pears to provide a consistent difference. In 
all examined juvenile specimens of Marmo- 
sops (fig. 7, upper panel) this tooth is fully 
molariform because the trigonid includes a 
distinct paraconid, protoconid, and metaco- 
nid. By contrast, the dp3 of juvenile Mar- 
mosa (fig. 7, lower panel) is not fully mo- 
lariform because the bicuspid trigonid lacks 
a distinct metaconid. 
COMPARISONS WITH GRACILINANUS: Museum 
specimens of Marmosops are sometimes mis- 
identified as Gracilinanus, apparently the re- 
sult of using inaccurate published keys.° De- 
spite such confusion, the two genera can be 
distinguished unambiguously by several ex- 
ternal morphological differences (table 2). 
Unlike Marmosops, which has a mesaxonic 
manus with dIII longer than the other digits, 
Gracilinanus (like Marmosa) has a paraxonic 
manus with dIII and dIV subequal in length. 
Also, whereas Marmosops has spirally ar- 
ranged caudal scales, the caudal scales of 
Gracilinanus are arranged in predominantly 
> Among other characters erroneously used in relevant 
key couplets by Hershkovitz (1992: 6) and Anderson 
(1997: 30), presence/absence of gular glands, plantar pad 
morphology, nasal proportions, and presence/absence of 
postorbital processes do not reliably distinguish species 
of Marmosops from Gracilinanus. 
