The loop configuration, structure and growth are studied in a series of megathiridoid dorsal valves ranging in length from 0·7 to 6 mm. The structure of the posterior and anterior sectors of the descending lamellae and their relationships with the socket ridges, dorsal septa and dorsal valve floor, are studied, illustrated and described. Detailed studies show that internal structures display great variability. In the megathiridoids the lamellae of more typical terebratulide loops are replaced by partially developed loops in early developmental stages, resulting in a mix of free sections of loop together with sections where the lophophore rests directly on the epithelium of the valve floor. Comments on the ecology of the megathiridoid species are included and relate their unusually wide gapes, which position their lophophores fully accessible to the open sea, with their strong lophophore attachments and loss of typical free loops. The new genus Joania (type species Terebratula cordata Risso, 1826) is erected for those Argyrotheca which, although having a typical megathiridoid brachidium, differ in their adult crural development, their narrow hinge line, their prominent cardinal process, their characteristic dorsal median septum and their tuberculate radial ridges terminating anteriorly in tubercles.
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