There are two native species of Microcitrus (now re-classified as Citrus) which are found in Papua New Guinea. They are closely related to the Australian Fingerlime (Microcitrus or Citrus australasica).  The first species is known as (Micro)citrus wurburgiana, but specimens from a second species were collected in 1970. Described in the journal 'Baileya. A quarterly journal of Horticultural Taxonomy' in 1976 this species was first named Microcitrus papuana, but David Mabberley of the Royal Botanic Gardens in Sydney, has recently re-named it Citrus wintersii after its collector.
As I was given some seeds said to be of this variety, I have taken a particular interest in it. The only published information seems to be the original 1976 description from which the quotes and drawings below are borrowed.
"A curious citrus-like shrub, growing naturally in the transition area between eucalyptus savanna and tropical rain forest at the Forestry Station, Brown River, Central District, Papua New Guinea."

"Fruits cylindric, slightly curved, abruptly beaked at apex and abruptly narrowed at base, 5-8 cm long, 1.0-1.3 cm thick at broadest part, green to yellow at full maturity, 3-5-celled, each cell usually containing 5 seeds."

"Fruits with strong odor of limes when crushed." 

"Branchlets minutely pubescent when young"
Microcitrus papuana drawings
page updated 31 October 2005
'Minutely pubescent' (=tiny hairs) is said to be a distinguishing feature of this species, so I decided to check this with the help of my toy QX3 microscope.
I took a small shoot from my best ' Microcitrus papuana' seedling (left), still only about 10cms tall, but re-growing well after much winter die-back. With the naked eye, no hairs were visible.
The series of microscope pictures above (X10, X60, X200 magnification) clearly shows these very tiny hairs. So Microcitrus papuana possibly confirmed, although at this magnification perhaps other species also show such hairs. More checking required!
or Citrus wintersii
seedling Microcitrus papuana
shoot x10
shoot x200
shoot x60
Tintori Microcitrus papuana
A specimen labelled M. papuana in the Tintori Hesperidarium.
The plant is about 1m tall
Leaves show a distinct variation from linear juvenile to broader more mature leaves. This is a common feature of other microcitrus species, but is not mentioned in the original description of M. papuana, nor is it shown on any of the herbarium pictures shown above.
linear leaf
broader leaf
In 2003 I was given another specimen said to be M. papuana. This grafted plant, possibly also  originating from Tintori, is now (October2005) about 0.6m tall, and is just showing some very unusual flower buds.
This  is the sole survivor of a batch of seedlings grown from M. papuana seed sent to me from USA.
I am suspicious about the identification, because the leaves are more like M. auatralasica rather than the linear leaves of papuana.
Tintori papuana leaves detail
Enlargement of the leaves, shows some are broader, more willow-like rather than  completely linear. Possibly a hybrid?
So, perhaps these broader leaves are the result of using fertilizers on potted plants, or perhaps this is a hybrid, as suggested for the Tintori specimen.
Microcitrus papuana USNA1970
                                             R. Pullen        24 April 1967
Brown River forestry area, c. 22 miles N. of Port Moresby. Inside margin of gully rainforest adjacent to Eucalypt savannah. Alt c. 120 feet.
Spiny shrub c. 6 feet tall, though usually 3-4 feet. Often 3-4 stems arising from a common rootstock at ground level. Crushed leaves with a distinct citric smell. Flower buds cream, single in the axils. Flowers with pale green calyx, 5 white petals, c. 20 stamens. Anthers yellow on white filaments. Pistil yellow-green. A single fruit found, green, elongated round in x-section and rather beaked apically.
P.van Royen   24/1/63
Shrub in undergrowth of rainforest, up to 5 feet high. Leaves dark green. Flowers pale yellow. Fruits dark green.  Brown River, Central District, Papua.
P.C.Heyligers  18/6/65
I mile SW. of Edebu, Brown River
Eucalypt-savanna remnant on top of hill.
Spiny low shrub, up to 0.5m tall.
(Only one bud found).
H. F. Winters & J. J. Higgins
26 March 1970
Department of Forests, Brown River.
Shrub to 1.5m; leaves tiny; slender, spiny twigs; fruit 1.3cm X 6.35cm, yellow at maturity. Wild.
K. J. White 30/5/74
Brown River Central District

Habitat: Forests
Habit: Shrub, 2m.
Microcitrus papuana CANB65
Microcitrus papuana +fruitCANB63
Microcitrus papuana CANB67
Microcitrus papuana CANB63
Microcitrus papuana USNA1974
There are herbarium specimens in Canberra, Australia and Washington, USA. The first four pictures below are details from the Canberra Herbarium collections. The last two are from the U.S. National Arboretum Herbarium. Texts are copied from the herbarium sheets. Colours are not true to the living plant.    All pictures reproduced with permission.
Type Specimen

This plant died over winter 2004
Citrus papuana flower bud
Microcitrus papuana flower buds
Microcitrus papuana tiny flower
These images are greatly enlarged, the smaller buds being only 1mmx2mm, the largest 3mmx6mm.
The tiny buds emerge entirely light green and round. The stigma and immature fruit then pokes out of  thin separated petals which turn from green to white. More photos will follow developments!

I would be very interested to hear from anyone else about their experiences of this species.
Please email me!