Shell large and solid, up to 37.6 mm in length. Most adults measure between 20 and 15 mm in length. Quite high in shape and with a straight outline when seen from aside. The perforation is large and has the typical keyhole shape: a long split enlarged in the center and at both ends.
There are about 44 to 50 strong radiating ribs descending from the top of the shell to the periphery. They are crossed by 22 concentric ribs on the big slope in the holotype. Where the weak concentric ribs cross the radiating ribs, there is a knob formed, strong, almost in the shape of pearls in some areas.
The interior callus shows the imprint of the radiating outer ribs: they are seen as a pale striae on photographs. In the holotype there are 58 such lines. It concerns the backside of the about 50 strong radiating ribs, and an additional number of secondary ribs. The keyhole is surrounded by a thick glossy white callus with the typical Diodora shape. The periphery, seen from the inside is deeply incised below the ending of each big radiating rib. On top of that, there is a small incision in the center of what appears as small dents. We refer to the photos for details. The coloration in fresh shells is all in greens: the pale green base color is adorned with darker areas, set in concentric zones, and there are usually 6 broad much darker radiating bands set in a perfect symmetry over the dorsum. One band ends in the center of the backside, one band ends in the center of the front, and 4 smaller bands radiate to the sides, 2 on each side. The interior color is white, but some shades of the outer pattern shine through the shell. We refer again to the photos for a general under-standing of shapes, sculptures and coloration.

We here compare the species with other Indo-Pacifk Diodora with which it was confused in the past, or that bear some similarities.


Diodora foveolata (Garrett, 1873)
This Pacific species, only known from 3 dead collected pieces in the Philippines is much smaller in size and has much more pronounced ribs compared to D. occultata n. sp. It also lacks the solid green coloration and a pronounced pattern in the shape of a cross. In D. foveolata, secondary ribs are more numerous and more visible than in D. occultata n. sp.


Diodora lincolnensis Cotton, 1930
Is an Australian species also reported from In-donesia. It has been illustrated only 3 times in the classic literature: in Wilson (1993), Kaicher, and
Dharma (2005). The species can be distinguished at once by its larger size (up to 60 mm), and by the presence of 3 primary ribs on the frontside - the big slope - of the shell. Such primary ribs are absent in the new species which is also more slender in outline. Compared to a set of 20 D. lincolnensis in the Coll. G. T. Poppe, D. lincolnensis has not the bright green coloration of D. occultata n. sp.: it is also most often greyish tinted, has a much rougher sculpture and is much bigger indeed.


Diodora ruppellii (G. B. Sowerby I, 1835)
Is mainly a Red Sea species, but apparently spreads along the East African coast. It is readily distinguished from the new species by its much smaller size, and the presence of a more contrasting color pattern, often with black rays. Its shape is also more slender and green is a rare coloration in D. ruppellii. The sculpture is less pronounced.


D. occultata n. sp. has been wrongly identified since the first specimens appeared in tangle nets and dredgings from deep water in the late 1970's. It is a large species, but it lives deep. CONCH obtained 40 pieces, mainly between 2003 and 2010, the latter date is the moment Balicasag and Aliguay stopped fishing in deep water. We suspect that over a hundred pieces must be in collections worldwide by now.
"Diodora occultata", or "the hidden Diodora". Hidden because it has been handled between collectors and scientists under a variety of wrong names.

TYPE LOCALITY
Philippines. Balicasag Island. 100 to 250 m deep.
DISTRIBUTION
At present only known from the center to the far south of the Philippines: Balicasag Island, Aliguay Island, Olango Island, Balut Island, Masbate and Palawan. This is a rather deep water species which usually was either dredged or tangle netted between 50 and 250 m, but the majority of shells come from below 100 m.