PYC>*ODO>'TIDX. 
199 
Genus MESODON, Wagner \ 
[Abh. k. bay. Akad. Wiss., math.-phys. Cl. vol. vi. 1851, p. 56.] 
Syn. (P) Typodus, F. A. Quenstedt, Der Jura, 1858, p. 781. 
Trunk discoidal, not produced at the caudal pedicle. Head and 
opercular bones externally ornamented with granulations; cleft of 
mouth very oblique; teeth smooth, or with feeble indentation and 
rugae; vomerine teeth arranged in five longitudinal series, the 
lateral pairs being often irregular; splenial dentition comprising 
one principal series of teeth with three or more outer series and 
one or two inner series, usually irregularlv arranged. Xeural and 
haemal arches of axial skeleton not sufficiently expanded to meet 
round the notochord. Fin-ravs robust, closely articulated, and 
much divided distally. Pelvic fins present: dorsal and anal fins 
deep throughout their extent, the former occupying the hinder half 
of the back, and the latter somewhat shorter; caudal fin fan¬ 
shaped, having a convex hinder margin, and arising immediately 
between the posterior extremities of the dorsal and anal fins. 
Scales robust, ornamented with granulations, and occupying only 
the anterior half of the trunk. 
Of this genus many species are known only by the dentition, and 
in that case they cannot be satisfactorily defined. The form of the 
X 
teeth is especially variable, but the number and relative proportions 
of the several series appear to be sufficiently constant for noting in 
provisional definitions. 
Mesodon macropteras (Agassiz). 
[Plate XVI. fig. 1.] 
1834-44. Gyrodus macropterus, L. Agassiz, Poiss. Foss., Feuill. p. 18. 
and vol. ii. pt. ii. p. 301. 
1851. Mesodon macropterus, A. Wagner, Abh. k. bay. Akad. Wiss.. 
math.-phys. Cl. vol. vi. pp. 49, 56, pi. iv. fig. 2. 
1856. Mesodon macropterus, J. J. Hec-kel, Denkschr. k. Akad. Wiss.. 
math.-naturw. Cl. vol. xi. p. 202. 
1862. Mesodon macropterus, A. Wagner, Abh. k. bay. Akad. Wiss., 
math.-phys. Cl. vol. ix. p. 349. 
1887. Mesodon macropterus, K. A. von Zittel, Handb. Paleeont. vol. iii. 
p. 247, fig. 261. 
Type. Xearly complete fish ; Palaeontological Museum, Munich. 
The type species, attaining a length of about 0*16. Maximum 
1 This name having gained universal acceptance, we adopt it. notwith¬ 
standing its preoccupation among Moliusea by Mesodon, Rafinesque. 1519. 
