Although the larva of Porcellana is one of the earliest known decapod larvae and at a later date the different stages of P. longicornis were well described by Sars (1889), there is yet much that is unknown. Few larvae have been actually hatched from the egg and in no case has the number of larval stages been definitely ascertained. In order to fill in some of the gaps in our knowledge a detailed study of the two common Plymouth species was begun (P. longicornis and P. platycheles). Unfortunately, war conditions made it difficult to obtain material and many specimens were lost as the result of air-raids; it was therefore not until the summer of 1942 that the work could be finished.

It is now possible to distinguish between the two species in all stages. Both have been hatched from the egg, larvae from the plankton have moulted and the post-larvae have been obtained from the last larvae. A comparison with Gurney's (1938a) notes on the species from the Red Sea and his general comments on the larvae of the genus and its relatives (1942) is of much use. These, with the first larvae of Petrolisthes armatus hatched by myself at Bermuda and certain larvae described by other workers, have made it possible to make some progress in the elucidation of the relationships of the larvae.

Porcellana longicornis and P. platycheles are both common at Plymouth; but whereas the latter is confined to coastal waters and lives between tidemarks and down to about 3 fathoms, the former has a much wider range and although occurring withP. platycheles between tidemarks extends to well beyond the Eddystone grounds.