. Carnegie Institution of Washington publication. JURASSIC 139 regarded as belonging to the head. It was noted, further, that the pecuHar fibro- cartilage spines, characteristic of Menaspis, may be interpreted as homologous with the so-called lip cartilages of the later Squaloraja; on the other hand, the paired head spines of Menaspis correspond with those later seen in Myriacanthus, although, naturally, they were less highly specialized. If, accordingly, Menaspis proves to be a Permian Chimaeroid, it certainly simplifies the problem of Chimaeroid descent. It indicates a shark-lik
. Carnegie Institution of Washington publication. JURASSIC 139 regarded as belonging to the head. It was noted, further, that the pecuHar fibro- cartilage spines, characteristic of Menaspis, may be interpreted as homologous with the so-called lip cartilages of the later Squaloraja; on the other hand, the paired head spines of Menaspis correspond with those later seen in Myriacanthus, although, naturally, they were less highly specialized. If, accordingly, Menaspis proves to be a Permian Chimaeroid, it certainly simplifies the problem of Chimaeroid descent. It indicates a shark-like form having four dental plates (fig. 115), like Deltodus or Sandalodus, and a dermal armoring which advanced pari passu with the develop- ment of the Figs. 126-131.—Associations o{ denial plates in fossil Chimaeroids. Lateral aspect. 126. Ptyctodus: 127. Rhynchodus secans (attached to the mandihular denial plate is shown the outline of the entire mecketian caitilage) : 128. Palaeomylus greenei: 129. Paleeomylus crassus; 130, Myriacanthus paradoxus; 131, Ischyodui. JURASSIC CHIM/EROIDS. Our definite knowledge of early Chimaeroids does not, however, begin before the Lias (Lower Jurassic), when remains of Squaloraja and Myriacanthus occur, notably in the fine-grained limestones of Lyme Regis. In this favorable matrix Squaloraja is so perfectly preserved, even in its cartilaginous parts, that we are enabled to reconstruct its essential characters. As shown in fig. 138, it appears as a somewhat flattened form; its vertebral column is strengthened with fine, closely set, ring-shaped thickenings which resemble those of a typical recent Chim- seroid ; the cranium is autostylic (Traquair) and bears in the male the frontal clasping organ, which here is long and spine-shaped (figs. 137, 137 A, 138, and detail in fig. 139), situated immediately in front of the eyes and folding forward. The orbits are large, and between them the breadth of the cranium suggests that the. Plea
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