This definition very properly excluded the oysters which had been erroneously inserted in this genus. Only the _Mytili_ and _Modiolæ_ of Lamarck were left as members; for _margaritiferus_ and _hirundo_ were transferred to _Ostreum_!! M. FRONS. _Ost._ acutum plicatum, labio altero scabro. M. CRISTA-GALLI. _Ost._ acutum, plicatum, labio utroque scabro. The reading of the MS. was not "secundum marginem insculptus," but "secundum marginem Mytilus." M. HYOTIS. _Ost._ subacuto-plicatum imbricatum squamis compressis, labio utrinque glabro. M. MARGARITIFERUS. _Ost._ semiauritum, imbricatum tunicis, basi transversum. "Bonan. 2. f. 1." was the omitted synonym. The description of the hinge was not at first inserted. M. UNGUIS. _M._ subrotundus, longitudinaliter striatus, pellucidus. This ambiguous species was not named, but placed next to _Ostrea edulis_. I entertain but little doubt of its being a young _Perna_. M. LITHOPHAGUS. _M._ cylindricus. Neither Gualtieri nor d'Argenville was referred to, which confirms my idea that the species of the 'Museum' (termed _coriaceus_ in the manuscript) was not the Mediterranean _Lithodomus_. M. BILOCULARIS. _M._ striatus, cardine fornicato. M. EXUSTUS. _M._ striatus, dorso angulato. M. EDULIS. _M._ lævis, subcurvatus, cardine terminali mutico. The printed synonyms are additions. From the "crassa," and the "absque denticulo," it is by no means improbable that some large exotic species was intended. The proposed name was _niger_; and that word originally formed part of the heading, but had been erased by Linnæus. M. UNGULATUS. _M._ læviusculus, valvis obliquis postice dilatatis, antice apice. "Lineis" was "tunicis" in the original. I do not consider that the details of this species (the _M. rusticus_ of the MS.) pertain, even generically, to the _ungulatus_ of the 'Systema.' The large _Mytilus_ represented by Gualtieri was not quoted: the reference, on the contrary, was to the two _Modiolæ_ depicted by Rumphius ("Rump. 151. t. 46. f. B. C.") and to their Kleinian names ("Klein, 127. Musculus acutus vulgaris, a. b."); and to that genus, rather than to _Mytilus_, does the account of the suture, and the final remark, apply. M. MODIOLUS. _M._ lævis, cardine sublaterali, margine dorsali dilatato. The erroneous, yet approximate, synonyms of Rumphius and Gualtieri (the 4 H's of whose engraving represent 4 different shells) were not quoted in the original. The species (for want of a good figure) was not clearly defined until the twelfth edition of the 'Systema.' M. VIRIDIS. _M._ lævis membranaceus, cardine terminali. M. RUBER. _M._ rugosus, valvulis obliquis, postice dilatatis, margine antico apicem æquante. The preceding mussel spoken of was not _viridis_, but _ungulatus_. The reading was not "brevissimo, compresso," but "brevissime compresso." M. HIRUNDO. _Ost._ valvis bilobis, lobo anteriore angustiore longiore. "Bonan. 2. f. 57" (error for 58) and "List. 220. f. 55" were quoted. PINNA.