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44 | / CHIM-EEOIDEI. 



Rostral cartilage somewhat produced, bearing a terminal cutaneous 

 flap. Mandibular tooth more or less massive in external appearance, 

 though really a thin plate ; symphysial surface narrow ; oral surface 

 undulating and covered by an extended, punctate, tritoral area, 

 almost or quite continuous. Presymphysial tooth vertically elon- 

 gated, bilaterally symmetrical, compressed antero-posteriorly, the 

 inner aspect being flat or concave, the outer aspect convex. Pala- 

 tine tooth thin, plate-like, triangular or irregularly quadrate in 

 form, the outer margin being nearly straight, sharply deflected and 

 thickened, the inner and posterior margins tapering gradually to a 

 thin edge ; oral aspect with a continuous, punctate, tritoral area. 

 Vomerine tooth smaller than the palatine, of triangular form, broad 

 posteriorly, and provided either with a long anteriorly-directed 

 process or with a distinct small tooth in front ; punctate tritoral 

 area subdivided into rounded patches. Dorsal fin-spine long and 

 slender, somewhat laterally compressed, with a large internal cavity ; 

 sides more or less ornamented with small tubercles ; a series of 

 large, thorn-shaped, spinous tubercles arranged along each edge of 

 the flattened posterior face, passing into a single median row dis- 

 tally, and a single series of similar denticles occupying at least 

 part of the anterior border. Rostral spine of male elongated and 

 pointed, with expanded base. Dermal plates tuberculated. 



Myriacanthus paradoxus, Agassiz. 

 [Plate II. figs. 1-3.] 



1822. "External defensive organ," H. T. De la Beche, Trans. Geol. Soe. 



[2] vol. i. p. 44, pi. y. figs. 1, 2. 

 1837. Myriacanthus paradoxus, L. Agassiz, Poiss. Foss. vol. hi. p. 38, 



pi. vi. 

 1837. Myriacanthus retrorsus, L. Agassiz, ibid. p. 39, pi. viii a. figs. 14, 



15. [Base of spine ; Oxford Museum.] 

 1843. Chimcera (Ischyodon) johnsonii, L. Agassiz, ibid. p. 344, pi. xl c. 



fig. 22. [Dentition ; British Museum.] 

 1872. Prognathodus guentheri, Sir P. Egerton, Quart. Journ. Geol. Soc. 



vol. xxviii. p. 233, pi. vin\ [Dentition ; British Museum.] 

 1889. Myriacanthus paradoxus, A. S. Woodward, Ann. Mag. Nat. Hist. 



[6] vol. iv. p. 279. 



Type. Dorsal fin-spines ; British Museum and Bristol Museum. 



The type species, of large size, the dorsal spine attaining a maxi- 

 mum length of not less than 0*6. Dorsal spine oval in section, 

 flattened posteriorly, and with a faint anterior longitudinal ridge : 

 lateral tuberculations relatively large and sparse, arranged on a 

 longitudinally striated surface ; anterior and posterior denticles very 



