. L'Bashford Dean memorial volume :. Poissons, requins, poissons, combustibles. 346 Bashford Dean Memorial Volume chaque rangée, les dents ont été formés par l'union des denticules simples homologues avec écailles placoïdes. Aux extrémités des lignes, les dents étaient petites et rudimentaires ; chaque dent avait de un à trois cuspides. Ceux avec un seul point portait un fort resem' blance à écailles placoïdes. Dans les dents avec deux ou trois cuspides, les cuspides étaient si étroitement soudés à leur base que l'émail a été continu d'un point à un autre. Selon Rose, ces dents représentent un stade transi' au

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. The Bashford Dean memorial volume :. Fishes; Sharks; Fishes, Fossil. 346 Bashford Dean Memorial Volume each row, teeth were being formed by the union of simple denticles homologous with placoid scales. At the outer ends of the rows, the teeth were small and rudimentary; each tooth had from one to three cusps. Those with a single cusp bore a strong resem' blance to placoid scales. In the teeth with two or three cusps, the cusps were so closely fused at their bases that the enamel was con- tinuous from one cusp to another. According to Rose, these teeth represent a stage transi' tional to the adult teeth of many teleosts. Possibly these teeth were anomalous, since in my four large specimens the outer teeth are only slightly different from those at the middle of each row: all have three cusps well devel' oped and well separated. Rose thinks that all the two' and three-cusped teeth of his embryo developed through the fusion of simple cusps. On one side of the upper jaw of his em- Teeth of the lower jaw (x 5) of a 340-mm. embryo ^ R5se found the first two teeth of the of Chlamydoselax:nus, in their natural positions. i . i ⢠i i ⢠i i i i- â i ., â third row united at their bases, but delimited After Rose, 1895, Abb. 5. , , yr^ r r ⢠x ^ by a deep groove (Text'figure 12 herein). One of these teeth has but one cusp, the other has two cusps. Rose claims that this anomaly has a phylogenetic significance, since it indicates the manner in which a jawbone might arise through the fusion of teeth at their bases. Further, Rose asserts that the three- and especially the five-cusped teeth of an adult Chlamydoselachus furnish an excellent transition between a single-cusped shark tooth and the toothplates of an adult Siren, likewise of all urodele embryos. Also, he finds in his Chlamydoselachus embryo all possible forms intermediate between a simple placoid scale and a three-cusped tooth. The single-cusped tooth shown at the left in Text-figure 12 differs very little fro

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