Archive image from page 155 of Devonian fishes of Iowa (1908). Devonian fishes of Iowa . devonianfishesof00east Year: 1908 162 IOWA GEOLOGICAL SURVEY the Dipnoan type, and that veritable homologies exist between their cranial roofing plates and those of the living Neoceratodus. This position is maintained in the following discussion of the group, as it seems to be most nearly in accord with the prepon- derance of known facts. Fig. 23. Fig. 23. Mylostomavariabile Newt). Cleveland shale; Sheffield, Ohio. Complete tritoral dentition arranged in natural position, but the elements of both pairs
Archive image from page 155 of Devonian fishes of Iowa (1908). Devonian fishes of Iowa . devonianfishesof00east Year: 1908 162 IOWA GEOLOGICAL SURVEY the Dipnoan type, and that veritable homologies exist between their cranial roofing plates and those of the living Neoceratodus. This position is maintained in the following discussion of the group, as it seems to be most nearly in accord with the prepon- derance of known facts. Fig. 23. Fig. 23. Mylostomavariabile Newt). Cleveland shale; Sheffield, Ohio. Complete tritoral dentition arranged in natural position, but the elements of both pairs of palato-pterygoid plates belonging to different individuals. The presence of a third pair of vomerine elements in advance of the two here shown has not yet been established by positive evidence, x 1-1. Concerning the systematic arrangement of Arthrodires, it need only be said that, owing to faulty preservation, we are still too imperfectly acquainted with the details of structural organization in different families to permit of more than a provisional scheme for illustrating successive stages of ad- vancement. Analogy with other, and especially higher, groups leads one to expect the dentition to furnish not only reliable clues as to relationship, but also a convenient and serviceable basis of minor classification. Experience proves that the ex- pectation is only partially justified. As between the two well- marked types of dental structure, which find close parallels in the existing Neoceratodus and Protopterus respectively, there need be little hesitation in recognizing the former, or triturating type, as the more primitive, it being one of the most constant, persistent and distinctive features of the Dipnoan stem. Nothing is more natural than to regard the trenchant or sectorial type
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