There is very little published information about this hybrid - probably between two of the Australian native citrus species - and its origin is a little confused.

The Citrus Industry, Volume 1, states: This remarkable hybrid was grown from seeds of the Australian round lime, Microcitrus australis, or of the finger-lime, M. australasica, sent to the U.S. Department of Agriculture's Agricultural Research Service by the late J. E. Maiden, director of the Botanical Garden at Sydney, Australia.

So, it would seem this was a single chance seedling from a mixed batch of seeds, and grown in the USA

Yet there is a tree of this variety growing in the Palm Grove in Sydney's Royal Botanic Garden. David Mabberley, at one time director of RSBG, reported some years ago that the tree was then in poor condition. He also said he has been unable to verify its exact origin - was it a re-introduction from the California specimen or another home-grown Australian plant?

In October 2013,  the gardens Information Officer told me "Our tree has been propagated by cuttings and seed and we have plants (grown from cuttings) established elsewhere in the Garden."
As far as I can ascertain, Saga Agricultural University in Japan is the only institution that has done any significant research into hybridization of the Australian native citrus varieties. A hunt around their website, using (for those unable to read Japanese) the Google Translate tool, used to reveal several more pictures of Microcitrus virgata and some details of further breeding work done with it. However, the website has now been changed and this information seems to have disappeared. I have therefore made available some of their photos - click the link below.
page created 22 Sept 2009, updated 4 Nov 2013
Microcitrus virgata -CVC1 modified
UCR-CVC Sydney Hybrid picture1
UCR-CVC Sydney Hybrid picture1
These photos of accession CRC #1485 were taken by the late Willard "Bill" Bitters , curator of the Citrus Variety Collection from 1947 to 1982.  Images from the original scanned slides were sent to me by UCR, and copyright remains with UC-Riverside Citrus Variety Collection
I have enhanced the quality of the original scanned slides, and derived these new images. Note that the picture above shows a metric scale ruler, the one on the right is in inches.
The fruits are about 2.5cms diameter, 5.0 cms long (1" x 2" ).
It is interesting that the fruits shown here contained several seeds. The original Citrus Industry description (and
repeated here), clearly says "seeds none".
Microcitrus virgata - CVC2 modified
However, at the time of writing in September, 2009, a further three plants are carrying fruit. Two of these do indeed look like the fruits in the original photos above, and the third is somewhat intermediate. So it seems to me that seedlings of Citrus x virgata are variable. Of-course, any seeds from Riverside are open-pollinated and surrounded by citrus trees of all species and varieties, so it is imposible to confirm the pollen parents of my plants.
The seedlings were all very thorny, but of variable vigour. In 2007, the first plants set fruit. They were entirely spherical - they looked just like the Round Lime, Microcitrus australis. So I thought perhaps there had been a mistake, and I was reluctant to describe the plants on this website as 'Sydney Hybrid'.
In early 1997 I was sent seeds from USDA-ARS National Germplasm Repository for Citrus and Dates, Riverside, California. They were listed as Sydney Hybrid CRC1485. In other words, they were seeds from the same plant as photographed by Bill Bitters.
Fruits from two seedlings Microcitrus xSydney
round fruit from untypical xSydney seedling
typical xSydney fruit
intermediate xSydney fruit
Fruit from the two similar seedlings above - typical Sydney Hybrids
Typical Sydney Hybrid
Intermediate fruit
Round fruit - like Microcitrus australis
Picture of the fruits from The Citrus Industry, Vol.1.
Sydney Hybrid seedlings
Here is a comparison between three of my  seedlings. The left plant is clearly more vigorous and produces spherical fruit.
The two right hand plants produce more elongated fruit.
Citrus Industry Vol1
This is the original tree still growing in the Sydney Botanic Garden. I visited Sydney in October 2016 and found the tree to be in reasonable condition, although tall and spindly and difficult to photograph due to surrounding trees and plants
Click here for further details from Sydney Royal Botanic Gardens and Domain Trust

...all names for the hybrid believed to be Microcitrus australis x Microcitrus australasica - Round Lime x Finger Lime

Saga University Hybrids