1. Study of the structure and mode of life of Malleus regula provides the basis for consideration of the "hammer" species, M. malleus and M. albus.
2. M. regula occurs byssally attached, vertically disposed on rocky substrates associated with mud and is widely distributed in the tropical Indo-Pacific.
3. The distal two-thirds of the elongated shell is exclusively prismatic. By means of special pallial retractors the mantle lobes can be withdrawn within the nacreous region.
4. The massive opisthodetic ligament has a short secondary extension of fusion layer.
5. A promyal chamber on the right side proximal to the adductor increases water flow into the exhalant chamber.
6. The long filibranch ctenidia provide a vertically extended food-collecting surface.
7. The foot is concerned with planting of the massive byssus which emerges through a notch in the right valve but on the under ( i.e., dorsal) surface. There is also a unique and very long accessory foot, ventrally grooved, everywhere ciliated and in constant writhing activity due to blood pressure and intrinsic muscle. Moving freely throughout the lower mantle cavity it can only be concerned with cleansing.
8. Pseudofaeces are ejected from the distal tip of the mantle cavity.
9. M. malleus, the black hammer shell, occurs vertically embedded in coarse sand or sandy gravel. Byssus threads are attached to fragments within the substrate. The great anterior and posterior elongations of the hinge line (also exclusively prismatic) are separated by the byssal notch (now affecting both valves).
10. Shells are usually excessively irregular due to the great exposure to damage and the almost unlimited powers of rapid repair by the three pallial extensions.
11. M. albus, the white hammer shell, is stouter and inhabits muddy sand. During growth it loses the byssus with reduction of the foot and byssal retractors and closure of the byssal notch. The accessory foot is not affected. The animal becomes anchored in the substrate exclusively by the anterior and posterior extensions of the shell.
12. Description of the typically rounded Isognomon ephippium leads to that of the elongated I. isognomon which occupies precisely the same habitat as M. regula.
13. It lacks the pallial retractors, promyal chamber and accessory foot of Malleus, the nacreous region is more extended distally and the ligament is multivincular (the formation of which is discussed), producing some posterior, but never any anterior, extension of the hinge line.
14. A remarkably dense mixed bed of M. regula and I. isognomon in Darvel Bay, Sabah (Borneo), is described. Different spawning periods, by preventing competition during settlement, may account for this complete intermixing of sympatric species, which, however, are generalized herbivores where numbers are not limited by food supply.
15. From a basic epifaunal habit (Pteria, Isognomon, M. regula), members of the Pteriacea have become adapted for infaunal life within sponges (Vulsella, Crenatula) or within soft substrates ( M. malleus, M. alba and the Pinnidae).
16. There is final discussion about elongation in monomyarians, i.e., in the genus Malleus and in I. isognomon.
1. Thirty-three discinid larvae were obtained one meter below the water surface in the daytime in the shallow part of the Johore Strait, north of Singapore. 2. The oldest among them, 462 µ in longitudinal diameter, resembled the "Mueller's larva" discovered by Mueller and referred to Pelagodiscus atlanticus (King) by many subsequent authors. 3. The youngest among them, 108 µ long and 115 µ wide in dorsal shell, resembled Simroth's smaller discinid larva of 220 µ diameter in many essential features. 4. The larvae in this study showed a gradual transition in many morphological characters, indicating that they were a series of developmental stages in the ontogeny of a single discinid species. 5. The two bunches of long embryonic setae were gradually replaced by the curved early larval setae in the medium-sized specimens. Towards the close of the larval stage the third kind of setae appeared from the mantle border. 6. The dorsal shell valve maintained its oval shape during the entire larval stage. The posterior edge of the ventral one, however, gradually changed from convex to concave, passing through a stage when it was almost a straight line. 7. The statocysts and the pedicle appeared in older larvae. 8. The number of cirri remained constant throughout the larval stage. 9. The larvae were compared with the discinid larvae previously recorded. Their developmental changes and the breeding season of discinids were discussed.