The larval shells of Paphies ventricosa, reared to settlement in the hatchery, are similar in longitudinal profile to shells of some other mactracean larvae; the umbo-stage larval shells are nearly round with the anterior margin less broadly curved than the posterior margin, the anterior end is longer than the posterior end, and the umbo is broadly rounded. Straight-hinge larvae had total lengths up to 149 |xm, and total heights up to 126 H-m, and umbo larvae had total lengths up to 320 H.m, and total heights up to 282 |xm. The larvae settled in about 22 days at a size of about 270 u,m. The larval hinge structure bears some similarities to those of other mactracean larvae.

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Larvae of Mytilus cdulis aoteanus, Perna canaliculus, Xenosirobus pulex, and Modiolarca impacta were reared to settlement in the hatchery. A detailed comparative study, based on microscopy (optical and SEM) and morphometry, has been carried out on the four species. External features and the internal hinge structure of the larval shells of the four species show typical mytilid characteristics. Externally, umbo stage larvae of Modiolarca impacta are ovoid in the anterioposterior direction, shell heights are substantially less than lengths, umbones are low and broadly rounded, and the anterior end is more pointed than the posterior. Mytilus edulis aoteanus and Perna canaliculus umbo stage larvae are similar, initially ovoid in the anterioposterior direction and later more rounded with shell heights approximately equal to lengths, umbones broadly rounded. The shoulders of P. canaliculus larvae are higher and more angular than those of M. edulis aoteanus. Xenostrobus pulex larvae are round to ovoid in the dorsoventral direction. Shell heights are equal to or greater than lengths, umbones are knobby. Provinculum lengths and shapes are similar in Modiolarca impacta, M. edulis aoteanus, and P. canaliculus. Provinculum length is shorter and the shape distinctive in X. pulex. The provincular teeth number in larvae 200-299 |am long is 16-24 in Modiolarca impacta, 18-24 in P. canaliculus, 20-30 in M. edulis aoteanus, and 10-15 in X. pulex. The primary lateral hinge teeth in pediveliger stage larvae of P. canaliculus do not develop in the other three species.

The larval shell development of Paphies subtriangulata is described from larvae reared to settlement in the hatchery. Externally the larval shells of P. subtriangulata are very similar to those of Paphies ventricosa and, to a lesser extent, Paphies australis. Internally the hinge structures of the late-stage larvae of P. subtriangulata, P. ventricosa, and P. australis are sufficiently different to distinguish the species. The spatulate tooth and the peg tooth of P. ventricosa are absent in P. subtriangulata and P. australis; the ligament of P. australis is posterior to the provinculum and central in P. ventricosa and P. subtriangulata. Straight-hinge stage larvae of P. subtriangulata were 88-143 µm long and 67-117 µm high and umbo stage larvae were 125-265 µm long and 97-223 µm high. The larvae settled in about 17 days from a length of 230 µm.

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