JURASSIC CHIM^ROIDS. 



145 



Chimseropsis, a third Jurassic j:;;enus, is known only from the lithographic stone 

 (Kimmeridgian = Upper Jurassic) of Bavaria. It resembles M\'riacanthus — as far, 

 at least, as one can judge from fragmentar\' remains. It certainly had similar 

 mandibular plates and the presymphyseal chisel-shaped element. It was provided 

 with a similar frontal clasping spine and an elongated snout. It had also a series 

 of dermal plates, as in the former genus, and in addition its trunk was studded 

 with small, conical, radially-grooved denticles. 



■*=;■■.:, 





Fig. 141. — Myriacanthus granulatus. Detail of snout region. 



After sr>ecimen presented lo Jermyn Street Museum by Captain Ibbetson. As indicated in dorsal aspect, the snout is broad and thickly 

 studded with dermal tubercles. The frontal clasping spine appears somewhat in its relative position. The dental plates are dissociated. 



Fig. 142.— Mandible of Myriacanthus, viewed from in front. 

 Restoration after one of Eserton's specimens in the British Museum. 



Brief mention need only be made of the Jurassic genera Ganodus (fig. 121) 

 and Brachymylus, since these forms are known only by detached dental plates. It 

 is possible, however, that a more or less complete skeleton of Ganodus* is preserved 

 in the Museum at Northampton (Smith Woodward, 1892), and, in this event, its 

 structures closely resemble Ischvodus.f 



*This specimen, a male, lacks the rostrum, but shows the frontal clasping organ ; of the latter the base is expanded 

 transversely, and shows, as in Myriacanthus and Squaloraja (figs. 134 a and 137 a), a faint median crest on its attached 

 face; its sides are laterally compressed. The column shows ring thickenings. Its dorsal spine is slender and arched 

 (= Lepracanthus). 



fSince the foregoing was written I have reexamined the specimen of " Ganodus " ai'i/us in the Munich Museum, 

 and am inclined to agree with its determination as Ischyodus by Reiss and Smith Woodward. It is quite possible, 

 however, that this specimen will be shown to represent a new genus as soon as a more definite knowledge of Ischyodus 

 is obtained. Thus the present specimen has small orbits, small snout, and large dermal denticles, the latter scattered 

 widely, especially conspicuous in the region just anterior to the ventral fin. There is also ground for the belief that a 

 pair of dermal plates were present on or near the posterior rim of the mandible. 



