The Quarterly journal of the Geological Society of London . hinge-line ; thedorsal beak is rather less pro-minent. The valves are almostequally convex; the ventralsometimes slightly more sathan the dorsal. Both valves-have a more or less pronoun-ced, median, rapidly-widening^sinus, bounded on each sideby rounded lateral folds whichdiverge from the beak andpass to the anterior margin,where they meet similar folds of the opposite valve. The junc-tion of the valves lies almost in one plane, or is very slightlysinuous. The shell-structure is silky and fibrous, the fibres beingmostly directed towar
The Quarterly journal of the Geological Society of London . hinge-line ; thedorsal beak is rather less pro-minent. The valves are almostequally convex; the ventralsometimes slightly more sathan the dorsal. Both valves-have a more or less pronoun-ced, median, rapidly-widening^sinus, bounded on each sideby rounded lateral folds whichdiverge from the beak andpass to the anterior margin,where they meet similar folds of the opposite valve. The junc-tion of the valves lies almost in one plane, or is very slightlysinuous. The shell-structure is silky and fibrous, the fibres beingmostly directed towards the median line. In a cast that I collectedin Carnic beds near Gore, in the Hokonui district, the beak hasbeen pierced by a narrow and long arched tubular foramen, butin others, and especially in the larger and more fully grownspecimens, the foramen is filled up and obliterated by the shellythickening of the beak. The delthyrium is triangular^and sunken,and in the larger specimens at least is closed. 1 Bibliography, 4, p. 273 & pi. xxix, %s. a=Dental sockets. 6 = Cardinal process of the dorsal valve,c = The recvirving- of the primary = The medio-laterally directed spiralia. 284 DE. C. T. TEECHMAFIS^ OS [vol. IxXlii, The following account of the internal structure is based chiefl}on large specimens of Hectoria tumid a, from the Rhaetic ofBenmore Cutting, where fine moulds and casts occur, from which Iobtained gutta-percha impressions. The interior of the ventralvalve is considerably thickened posteriorly by deposition ofshelly matter; the hinge-teeth are prominent, are supported byshort dental plates, and there is a short median septum. In the dorsal valve the hinge-region is still more thickened;the cardinal process is very large and prominent, and consists of ablunt tongue-shaped mass of shell, which projects backwardsbeneath the delthyrial region of the ventral valve. The cardinalprocess has a shallow depression on its surface. In front of the?cardinal p
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Keywords: ., bookcentury1800, bookdecade1840, booksubjectgeology, bookyear1845
