Many schemes of classification have from time to time been propounded, based upon the presence or absence of respiratory siphons, the number and position of the adductor shell-muscles, the character of the shell-hinge, &c. The most recent arrangement is founded principally upon the structure of the gills. The value of such a classification has yet to be fully tested. Mr. Paul Pilseneer has suggested five orders of Lamellibranchs: Protobranchia, Filibranchia, Pseudolamellibranchia, Eulamellibranchia, Septibranchia. [Illustration: Fig. 25. (From the ‘Cambridge Natural History.’ Messrs. Macmillan & Co.) A. Protobranchia. B. Filibranchia. C. Eulamellibranchia. D. Septibranchia. _m._ Mantle, _v._ Body. _f′._ Foot. _e._ Outer gill-lamella; _i._ Inner gill-lamella; _e′._ Reflected portion of outer lamella; _i′._ Reflected portion of inner lamella; _s._ Septum-like gill. ] [Illustration: Fig. 26. Gill of _Mytilus edulis_.[7] A. Part of four filaments showing ciliated interfilamentar junctions (cj). B. Diagram of a single filament showing the two lamellæ connected at intervals by interlamellar junctions (ilj) and the position of the interfilamentar ciliated junctions (ep). ] PROTOBRANCHIA. (Fig. 25, A.) [Sidenote: Case 137.] In this order the filaments of the gills are not reflected, but arranged in two divergent rows, the foot being expanded and flattened beneath with crenulated margins and with the byssal gland very slightly developed. The _Nuculidæ_ and _Solenomyidæ_ are the only families belonging to this order. The shells of the former are remarkable for the numerous fine interlocking hinge-teeth, and those of the latter on account of the strong fringed periostracum. FILIBRANCHIA. (Fig. 25, B.) [Sidenote: Cases 137–145.] In this group the gills are smooth, with the filaments directed downwards, reflected, and connected one with another by interfilamentar ciliated junctions, but the lamellæ are not connected. The foot is usually provided with a well-developed byssal gland. _Anomia_, _Arca_, _Trigonia_, and _Mytilus_ belong to this order. [Sidenote: Cases 137–138.] The family of _Anomiidæ_ contains a number of more or less pearly shells remarkable for a deep notch or hole in the lower or flat valve through which a shelly plug passes, by means of which the animal attaches itself to other shells, stones, &c. _Anomia ænigmatica_ is found adhering to leaves in mangrove-swamps. [Sidenote: Case 138.] The _Placunidæ_, sometimes called Window-shells and Saddle-Oysters, are very flat pearly shells with a remarkable hinge, which consists of two long divergent teeth, like a =⋀=, to which the ligament is attached. The species are few in number, and inhabit sandy shores of India, China, and North Australia. [Sidenote: Cases 139–141.]