The following suppressed description of an unnamed Ark that was allied to, if it were not, _fusca_ or _barbata_, was found in the manuscript. The 'Museum' had been referred to in the synonymy of the latter in the 'Systema,' but the species was not mentioned in the published version. _Arca_ ovalis, compressiuscula, apicibus subcontiguis. Testa rudis, ferrugineo-fusca, longitudinaliter striata, striis quasi ex punctis callosis concatenatis, alternis striis majoribus ovatis, parum obliqua, minus lateribus gibba, rotunda absque angulis. Margo æqualis, edentulus. Apices recurvi fere tangunt se invicem. Rima dentibus minutissimis, antice longius extensa, nec recta. OSTREA. The very natural genus _Pecten_ was separated from the unsymmetrical oysters, with the following definition:-- PECTEN. Testa subrotunda, altera planior, basis transversa, anguli transversi (auriculæ) ad basin. Cardo cavitas conica, striis utrinque 3 longitudinalibus obliquis. The genus OSTREUM, enlarged by the addition of the true oysters confounded with the _Mytili_, the _Meleagrinæ_, _Aviculæ_, and the _Anomia placenta_, was thus characterized:-- O. MAXIMA. _Pec._ radiis 14 rotundatis longitudinaliter striatis. In place of Gualtieri, "List. Ang. 184. t. 5. f. 29. Pecten maximus," and "Faun. Suec. 1343" were referred to: these synonyms had been added subsequently to the description. O. JACOBÆA. _Pec._ radiis 14 angulatis, fornicis longitudinaliter striatis. The cited drawing of Gualtieri was not mentioned. O. ZICZAC. _Pec._ radiis 18 explanatis. No specific name was attached to this, the preceding, and the next two species. O. STRIATULA. _Pec._ radiis 16 obliteratis, transverse membranaceostriatis, margine integerrimo. O. MINUTA. _Pec._ radiis 20 convexis. O. PLEURONECTES. _Pec._ radiis 12 duplicatis, extus lævis. O. OBLITERATA. _Pec._ radiis 24 duplicatis, extus lævis. O. RADULA. _Pec._ radiis 6 convexis decussate striatis, margine crenato, auriculis æqualibus. O. PLICA. _Pec._ radiis 16 convexis læviusculis, decussato-striatus. No specific name was appended to either this, the next, or the two preceding species. O. PALLIUM. _Pec._ radiis 12 convexis, striatus, scaber, squamis imbricatus. This with the remainder of the _Pectens_ (as far as _flavicans_) formed a group characterized by "Auricula altera intus ciliato-spinosa." O. NODOSA. _Pec._ radiis 9 nodoso-vesicularibus. O. PES-FELIS. _Pec._ radiis 9, lævis, fornice squamis fornicatis. The printed diagnosis, or heading, was evidently drawn up from a different shell. O. PELLUCENS. _Pec._ radiis 9, lævis, fornice squamis cochleari-hemisphæricis. No name was attached to this or the next shell. O. SANGUINEA. _Pec._ radiis 22 scabris, semiauritus. The reference was not to plate 74 (as printed) of Gualtieri, but to plate 73. "Purpureus nigro undatus" was written after the indicated colouring. O. VARIA. _Pec._ radiis 30 scabris explanatis.