JURASSIC CHIM^ROIDS. 



H3 



The second Jurassic Chimseroid, Myriacanthus, is known, unfortunately, in 

 less detail. Nothing has been definitely ascertained regarding its general shape or 

 the structures of its trunk. But what is known of its head region shows that it 

 possessed extraordinar}^ features. The form of the head was, in general, like that 

 of Callorhynchus, terminating in a long snout. This had a somewhat foliaceous 

 tip (fig. 140), as in the recent genus, but, on the other hand, was broader, less 

 acutely pointed, and studded dorsally with shagreen denticles and dermal plates. 



The best example of a snout of Myriacanthus belongs probably to a specimen 

 in the Jermyn Street collection, of which a sketch is given in figure 141. The 

 figure, which shows the snout in dorsal aspect, indicates also the spine-like nature 

 of the frontal clasping organ. This organ is shown again, in lateral view in fig. 133. 

 There can be little question that in this genus the shagreen-like defenses seen 

 in the head of Squaloraja are replaced by a number of conspicuous pairs of 

 dermal plates, some of which attain a large size and are furnished with spinous 

 outgrowths. Thus, for example, on 

 either side of the jaw (slightly schem- 

 atized in fig. 142) there is a conspic- 

 uous "trachyacanthid " spine bearing 

 a large serrate row of four or five 

 subspines. These elements, it may 

 be remarked, are well shown in a 

 second specimen from Lyme Regis, 

 in the Jermyn Street collection, and 

 in Egerton's type specimen of Piog- 

 nathodiis gucntheri (^AIyriaca)ifhits par- 

 adoxus), now preserved in the British 

 Museum, in which one of these 

 spines is shown in situ, attached to 

 the broad jaw. The arrangement 

 of the dental plates of Myriacanthus 

 is known with fair accuracy (fig. 119). 

 The mandibular plates show foldings on the visceral face and in these folded areas 

 appears the most conspicuous aggregation of tritoral points. A somewhat similar 

 condition prevails in the palatines. In front of the palatines, as in Squaloraja, 

 there occurs a pair of "vomerine" plates. These, however, instead of exhibiting 

 a finely arranged series of tritoral points, present three rows of larger tritors, 

 somewhat as indicated in the restoration (fig. 119 a). Furthermore, in front of the 

 "vomerines" (and this condition is unique among all other Chimseroids, fossil or 

 recent) there is a third and still smaller pair of plates, showing faintly a series of 

 rows of tritors. Another puzzle in the dentition of Myriacanthus is seen in the 

 region of the mandibular symphj^sis, for here occurs an azygous chisel-shaped 

 tooth which is known only in this genus and in the kindred Chimseropsis {cf. also 

 p. 145). The restoration in lateral view of these dental plates is shown in fig. 130. 

 On the other hand, Myriacanthus, like recent Chimseroids, was autostylic, and it 



fig- 



139. — Squaloraja. Detail ol rostial spine of specimen P 4323 

 in British Museum. 

 The dermal denticles are grouped closely together, their bases flat and greatly enlarged. 

 They occasionally become detached, as the scars in the specimen indicate. 



