A PYCNObONT FISH OF THE GENUS MESODON. 187 



of the back between the head and the dorsal fin, and the rays 

 are more numerous in the dorsal and anal fins. The only 

 comparable hump-backed forms hitherto discovered are M. 

 gibbosus* from the lithographic stone of Bavaria and France, 

 and M. granulatus, from the Corallian and Kimmeridgian of 

 Western Europe. f The first of these (which I once supposed 

 to be founded on a distorted specimen) differs from the Portland 

 fish in having the elevation of the back pointed, not rounded ; 

 but the second, so far as known from an imperfect skeleton 

 found in the Lower Kimmeridgian of Hanover, agrees very well 

 with the form now described in the round humped shape of the 

 back and in the nature of the ornamentation, including prickles 

 on the ventral scales. Moreover, the splenial dentition of M. 

 granulatus, as may be observed in an example from the Kim- 

 meridge Clay of Weymouth (PL B, Fig. 6), is remarkably similar 

 to that of the fish now described (PL B, Fig. 2). Unfortunately, 

 however, the median fins of M. granulatus have not yet been 

 discovered, and its tail is very imperfectly known, so that no 

 exact comparisons are possible. I, therefore, propose to regard 

 the Portland fossil now described as representing a new species, 

 M. barnesi, which may be defined as follows : 



Trunk raised into a rounded eminence between the occiput 

 and the dorsal fin, and its maximum depth nearly equalling its 

 total length to the origin of the caudal fin. External bones and 

 scales coarsely tuberculated, the dorsal and ventral ridge-scales 

 with enamelled prickles. Teeth smooth, a few of those of the 

 lateral series having a faint apical pit with traces of crimping 

 on the border ; splenial teeth closely arranged, those of the 

 principal series broader than long, flanked within by one row 

 of small round teeth and externally by three series, of which the 

 median is the smallest, and the outer about equal in size to the 

 inner series. Dorsal fin with about 30, anal fin with about 20 rays. 



* A. Wagner, Abh. k. bay. Akad. Wiss., math.-phys. Cl., Vol. VI. (1851), 

 p. 52, pi. iii., fig. 2. 



fK. Fricke, "Die fossilen Fische aus den oberen Juraschichten von Hannover," 

 Palseontographica, Vol. XXII. (1875), p. 359, pi. xviii., pi. xix., figs. 1-5. 



