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The visual system of the numbat (Myrmecobius fasciatus), a highly endangered diurnal marsupial, has been investigated both anatomically and behaviorally. The ganglion cell layer, examined in cresyl-violet stained wholemounts, contained 832,800 ganglion cells; the number of ganglion cells corresponded to optic axon counts. An area centralis was located in the mid-temporal retina, where cells formed a bilayer, but there was no evidence of a visual streak. Visual acuity, estimated from counts of peak ganglion cell density (8,100/mm2) and measurements of posterior nodal distance (7.84 mm), was found to be 6.3 cycles per degree. The value was similar to that of 5.2 cycles per degree estimated by behavioral tests. Sectioned material revealed the presence of numerous oil-droplets in a cone-dominated retina. A rich retinal vasculature of the end-artery type of paired arteries and veins formed beds in the ganglion cell and inner nuclear layers. Measurements of visual fields revealed a binocular overlap of 80° in the horizontal plane, and a total field of view approaching 240° of visual angle. Monitoring of pupillary responses with an infrared video camera showed that the numbat possesses a remarkably wide, static pupil. Our results are discussed in relation to the ecology of the species and its phylogeny.

The visual system of the fat-tailed dunnart (Sminthopsis crassicaudata), a small polyprotodont marsupial, has been examined both anatomically and behaviourally. The ganglion cell layer was examined in cresyl-violet stained wholemounts and found to contain a mean of 81,400 ganglion cells (SD ± 3,360); the identification of ganglion cells was supported by a correspondence to optic axon counts. Ganglion cells were distributed as a mid-temporally situated area centralis, embedded in a pronounced visual streak. Localised implants of horseradish peroxidase into retinal wholemounts revealed both A-type and B-type horizontal cells. Sections of the outer retina showed it to be rod-dominated, with a rod-to-cone ratio of 40:1 at the area centralis; cones were found to contain oil droplets but double cones were not a prominent feature. The retinal pigment epithelium consisted of squamous cells. Visual acuity, estimated from counts of peak ganglion cell density (8,300/mm2, SD ± 1,180) and measurements of posterior nodal distance (2.9 mm), was found to be 2.30 cycles per degree. The value was close to that of 2.36 cycles per degree estimated by behavioural tests using a Mitchell jumping stand; values were similar at low, intermediate and high light levels. Our findings are discussed in relation to the lifestyle of the dunnart.

We examined the retina and optic nerve of one of the smallest mammals, the honey possum (Tarsipes rostratus), a marsupial. Ganglion and displaced amacrine cells were distinguished by morphological cri