. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. General Classification of the Pelecypoda. 95 of alternating ridges and folds, called by liini " ; This band is separated into anterior and posterior portions by the primary liganicntal pit. In Ostren, howcvcr_, the anterior row is wanting, and tlie iigamental pit lies at the anterior edge of the shell. Subsequent to the development of the crenulations the true teeth make their appearance. These arise, not perpendicular to the hinge-line but as long ridge
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. General Classification of the Pelecypoda. 95 of alternating ridges and folds, called by liini " ; This band is separated into anterior and posterior portions by the primary liganicntal pit. In Ostren, howcvcr_, the anterior row is wanting, and tlie iigamental pit lies at the anterior edge of the shell. Subsequent to the development of the crenulations the true teeth make their appearance. These arise, not perpendicular to the hinge-line but as long ridges parallel to it. Tiiey may retain this position throagli- out life, as in CucuUea crassalina. Usually, however, the interior end becomes sharply curved and the external part atrophies, leaving the usual taxodont teeth (figs. 1, 2, and .'5j. Fig. I. Young CiiaiUea crn^xrttina, showing the recurving of the primitive hiniellte to form taxodont teeth. (After Bernard.) Iji = primary Iigamental teeth. The great importance of this is twofold :— Firsthj. It refutes the theory that the early embryonic dentition seen in certain Heterodonts, and which arises perpendicular to the hinge-line, represents an early taxodont condition, and for that reason necessi- tates the descent of the Heterodonts from Taxo- donts as seen in modern forms. Secondhj. It does away with the radical difference between the heterodont and taxodont teeth. 77^6 Origin of the Pleurodont Dentition. Amongst the Anisomyarians (figs. 4 & 5) the teeth show a still earlier stage than in the Nuculidre and their allies. Here they rise before the development of the cardinal plateau as lateral folds alternating with the external ribs. These internal ribs may occur where the outer test is smooth. In forms where the test thickens Ki'catlv a transition can be. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illust
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