The shell of the Pinnidae consists of characteristic fan-shaped valves united for their entire length dorsally by some form of ligament. The posterior and postero-ventral extensions of the valves are formed exclusively by the outer lobe of the mantle edge and so consist of the outer calcareous (prismatic) layer only. In the Pinnidae this layer has an exceptionally high organic content and so is flexible. The inner (nacreous) calcareous layer is thin and confined to the region occupied by the body, i.e. between the two adductors. The ligament is composed of three layers (disregarding the outermost, but vestigial, periostracum). Anteriorly it consists of inner and outer layers; posterior to this for a short distance of outer layer, and for the remaining, and greatest, length it represents a fusion layer. This also continues forward over the middle region and traces persist still farther forward. The inner ligament layer is secreted by the mantle isthmus, i.e. it corresponds to the inner calcareous layer of the valves; the outer ligament layer is secreted by the outer lobes of the mantle edge, i.e. it corresponds to the outer calcareous layer of the valves. The fusion layer represents the result of union posteriorly of the outer lobes of the mantle edge. That part of the ligament which is formed in the same way as the valves (all being constituents of the shell) is here termed the primary ligament, that part formed by fusion of the outer lobes of the mantle edge constituting the secondary ligament. The mantle epithelium is divisible into proximal and distal regions, both containing mucous glands. The former, continuous with the mantle isthmus, secretes the inner calcareous layer; the latter is pigmented. Apart from the adductors, the mantle is attached only by the large anterior and posterior pallial retractors which subdivide within the mantle folds. Posterior elongation of the mantle involves corresponding extension of the eulamellibranchiate ctenidia. All connexions between the two ctenidia and between ctenidia and mantle are by ciliary junctions. The ctenidia are very muscular; observations of Atkins on collection and sorting of particles are confirmed. The unique gutter-like waste canals, originally described by Stenta, ensure that pseudofaeces and other waste from the inhalant chamber are continuously removed. A preoral, unpaired racemose gland opens into the inhalant chamber. Its most probable function is that of excretion; anything discharged from it will be removed by way of the waste canals. The pallial organ in the exhalant chamber is composed of a stalk and a more swollen head. It can be greatly distended and probably serves to clear away shell fragments. The projecting valves are subject to frequent damage. The Pinnidae live vertically embedded in soft substrata into which they cannot withdraw. The animals burrow as they grow but only to the extent that the portion of the shell occupied by the body (i.e. as far as the position of the posterior adductor) is buried. The wide posterior region of the shell is always exposed. Water can thus be drawn in from well above the surface of the substratum. The waste canals in the inhalant chamber and the powerful exhalant current keep the cavity clear. The projecting valves can be rapidly repaired since they are composed exclusively of the outer calcareous layer. Such repair strengthens the valves. During repair new inner and outer ligament layers may be laid down beneath the previously formed primary ligament. In the Lamellibranchia change in form and proportions of the body on the one hand, and of the mantle and shell on the other, are best discussed by reference to the two major axes in the sagittal plane, i.e. the antero-posterior and median axes of the body and the hinge and normal axes of the mantle and shell. In their evolution the Pinnidae probably passed through a 'Modiolus stage' with the large posterior adductor close to the margin of the shell and little secondary extension of the ligament. Subsequently posterior extension of the mantle and so of the shell doubled the length of the animal and was accompanied by secondary extension of the ligament. Such extensions of the mantle occur throughout the Anisomyaria (including the Pinnidae). They involve loss of the primitive pallial attachments apart from the adductors (the anterior of which is always reduced and often lost). The mantle becomes re-attached to the shell by secondary pallial retractors in the Pinnidae and also in Malleus, but along a new line peripherally in the Pectinacea and Ostreacea. Existing data on development in the Pinnidae show that, with the formation of the dissoconch, new shell, secreted by the outer lobe of the mantle edge, is added to the posterior margin of the almost equilateral prodissoconch. The adult form is probably quickly acquired by the post-larva, the proportions of the different regions then remaining constant although with continual reduction anteriorly. Success in the Pinnidae is due partly to characteristics shared with related families, partly to unique features. The former include great extension of mantle lobes without peripheral attachment, the latter include waste canals, pallial organ and pallial retractors. The rigid ligament only unites the valves; it has no opening thrust. The Pinnidae can survive loss of the anterior adductor or fusion of the ventral margins of the valves. It is only essential that the posterior adductor should be able when necessary to pull together the flexible posterior margins of the valves.