"The first genus is found in the seas of warm climates. The shells are thin and foliated. The hinge of _Perna_ differs widely from that of the oyster. It is linear, formed of sulcated teeth. There is a sinus under the extremity of the hinge, for the passage of the byssus. Compare _P. isognomon_ with the common oyster, and you will find few points of resemblance between them. (Plate 5.) _Perna ephippium_ is also a curious species, very pearly within. The _hammers_ are rugged and singular in form. They are all foreign, from the oriental seas. "_Avicula_, or Swallow, so called from the resemblance of the shells to a bird flying, was considered as a single species by Linnæus. Lamarck makes eighteen species in his new genera. _Meleagrina_ has two species. The pearl-bearing muscle, as it is called, is found in the Persian Gulf, the Gulf of Mexico, &c. The interior of the shell is coated with thick pearl, and within it are formed those globular substances known by the name of pearls. * * * * * [Sidenote: MELEAGRINA. LIMA. PECTEN.] "Family, _Pectenida_: genera-- _Pedum_ Only species, from India, rare. _Lima_ _Ostrea_. _Plagiostoma._ _Pecten_ _Ostrea_, scallop. _Plicatula_ _Spondylus_. _Spondylus_ _Spondylus_, thorny-oyster. _Podopsis._ "The genus _Lima_ is longitudinal, auricled, or eared; hinge without teeth, with a hollow receiving the ligament. These are very pretty shells, generally white, almost transparent, resembling the _Pecten_. _Lima_ comes from the American seas, and is a species easily obtained. There are also several fossil species. "_Plagiostoma_ is wholly a fossil genus, of which several species are found in this country, in lias, &c. "The pectens are so easily known that I need only mention some fine species, such as _P. pallium_, a splendid shell, from the Indian seas: _P. pleuronectes_ is a finely polished, smooth species from the Indian Ocean. "The genus is divided into sections, viz. ears equal, ears unequal. You may find some common species on our own shores, and you may procure fossil species: they are numerous. [Sidenote: PLICATULA. OSTREA.] "_Plicatula_ is a genus taken from _Spondylus_. _Spondylus gæderopus_, from the Mediterranean, is a common shell in collections. "_Podopsis_ is a fossil genus. "_Ostracea._ "The oysters and pectens differ so widely that they do not even rank in the same family. * * * * * "In the _Ostracea_ are, _Gryphæa_ _Ostrea_. _Ostrea_ The same. _Vulsella._ _Placuna_ _Anomia_. _Anomia_ The same--Antique lamp. "There is but one recent species of the first genus; but many fossil.