JURASSIC CHIM.^ROIDS. 



139 



regarded as belonging- to the head. It was noted, further, that the pecuhar tibro- 

 cartilage spines, characteristic of Menaspis, may be interpreted as homologous with 

 the so-called hp cartilages of the later Squaloraja; on the other hand, the paired 

 head spines of Menaspis correspond with those later seen in Myriacanthus. although, 

 natural!}', they were less highly specialized. If. accordingly. Menaspis proves to 

 be a Permian Chimaeroid, it certainly simplifies the problem of Chim^eroid 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 dentition. 



Figs. 126-131. — Associations of denial plates in fossil Chimajtoids. Lateral aspect. 



126. Ptyclodus ; 127. Rhynchodus secans (attachwj to the mandibular denial plate is jhown the outline of the entire mecltelian caililage) ; 

 1 26, Paleeomylus greenei ; 1 29. Pala^omylus ciassus ; 130. Myriacanthus paiadoxus ; 131, Ischyodus. 



JURASSIC CHIM.'EROIDS. 

 Our definite knowledge of early Chima-roids does not, however, begin before 

 the Lias (Lower Jurassic), when remains of Squaloraja and Myriacanthus occur, 

 notably in the line-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. 1 38, 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. 1 39), 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 



