The encyclopdia britannica; a dictionary of arts, sciences, literature and general information . pedicle-opening seen in Orthis, &c., has beennamed by James Hall and J. M. Clarke the some less primitive genera, Terebratula, thattype of opening is found in the young stages only; laterit becomes partly closed by two plates which grow outfrom the sides of the delthyrium. These plates aresecreted by the ventral lobe of the mantle, and werenamed by von Buch in 1834^ the deltidium. the deltidium varies in different genera. Thetwo plates may meet in the middle line, and
The encyclopdia britannica; a dictionary of arts, sciences, literature and general information . pedicle-opening seen in Orthis, &c., has beennamed by James Hall and J. M. Clarke the some less primitive genera, Terebratula, thattype of opening is found in the young stages only; laterit becomes partly closed by two plates which grow outfrom the sides of the delthyrium. These plates aresecreted by the ventral lobe of the mantle, and werenamed by von Buch in 1834^ the deltidium. the deltidium varies in different genera. Thetwo plates may meet in the middle line, and leave onlya small oval opening near the centre for the pedicle,as in Rhynchonella; or they may meet only near thebase of the delthyrium forming the lower boundary ofthe circular pedicle-opening, as in Terebratula; or the. right plate may remain quite distinct from the leftplate, as in, Terebratella. The pro-deltidium, a term introducedby Hall and Clarke, signifies a small embryonic plate originatingon the dorsal side of the body. It subsequently becomes attachedto the ventral valve, and. develops into the pseudo-deltidium, in the Neotre-mata and the Protrem^ pseudo-deltidium (sonamed by Bronn in 1862)is a single plate whichgrows from the apex ofthe delthyrium down-wards, and may com-pletely close theaperture. The pseudo-deltidium is sometimesreabsorbed in the the Telotremataneither pro-deltidium norpseudo-deltidium isknown., In the Atrematathe pro-deltidium doesnot become fixed to theventral valve, and doesnot develop into a pseudo-deltidium. The American
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