Diagnosis
Shell subglobose, 13.5-23.9 mm in height and 18.0-28.4 mm in width. Shell pale greenish yellow to yellow with red umbilical patch and narrow red subsutural band. Inner lip pale pink to pale mauve. Mantle dark yellow. Penis and vagina long, about equal in length. Epiphallus short.
Description
Shell . Subglobose, 13.5-23.9 mm in height, 18.0-28.4 mm in width. Spire moderately elevated. Aperture roundly ovate, height 10.0-13.5 mm, width 10.8-18.7 mm. Total number of whorls 4.8-5.7. Last teleoconch whorl rounded, or with slight to moderate angulation; height 12.3-21.3 mm. Teleoconch sculpture of growth lines and weak pustules. Teleoconch periostracal sculpture of weak zigzag ridges. Protoconch sculpture weakly pustulose. Shell pale greenish yellow to yellow, with narrow red subsutural band. Red umbilical patch typically present, occasionally weakly developed to absent. Supraperipheral band absent. Inner lip pale pink to pale mauve, strongly reflected and largely obscuring umbilical depression. Outer lip moderately deflected below midline of last whorl. Umbilicus slightly open to closed. Dimensions of holotype: SH—16.9 mm, SW—22.8 mm, AH—12.0 mm, AW—14.7 mm, BW—14.4 mm, TW—5.50 whorls. Non-genital Anatomy. Animal dark grey to chocolate brown with yellowish speckles, mantle dark yellow. Kidney approximately 50-59% of length of mantle roof.

Since Meridolum middenense was originally described it has had a confused history. The type material of this species comprises two species. Both in the original description and in subsequent identifications, the name has been applied to most specimens, both recent and subfossil, collected from the sand dunes of New South Wales beaches from Port Stephens, south to Royal National Park. McLauchlan (1951) believed the subfossil shell deposits found on the dunes of most Sydney beaches were actually old shell middens left by the local aboriginal tribes, who he believed were utilising them as a food source. It is clear from the written description that McLauchlan (1951) considered the live-collected individual to be the type of M. middenense yet this specimen is identical in shell form to those found in the Hawkesbury Sandstone vegetation behind Mona Vale. These forest-dwelling individuals have a different shell shape and animal colour to the snails found in the leaf litter of the coastal heath and dunes. The specimen (= paratype) of M. middenense from Kurnell illustrated by McLauchlan (1951, figure 4) actually represents the dune-dwelling species, described in the present work as M. maryae.

Meridolum middenense is found under logs and leaf litter, in the Hawkesbury Sandstone vegetation of eastern Sydney, from the eastern edge of the Cumberland Plain Woodland to the coast. Meridolum middenense occurs in an area roughly between Port Hacking in the south to Broken Bay in the north. However, there are many coastal populations between Broken Bay and Port Stephens that are very similar, but pending additional field, anatomical and genetic work, these have been excluded.