222 
ACTIXOPTERTGII. 
with reticulating rugse and pittings; teeth smooth, sometimes 
feebly indented in the lateral series ; vomerine teeth in five longi¬ 
tudinal series, the inner lateral pairs regularly alternating with the 
widely spaced median teeth; splenial teeth in four series, the 
innermost being relatively small, and the second the largest or 
principal series. Neural and haemal arches of axial skeleton of 
trunk not expanding sufficiently to encircle the notochord. Fin- 
rays robust, closely articulated, and much divided distally. Pelvic 
fins present; dorsal and anal fins very high and acuminate in front, 
rapidly becoming low and fringe-like behind, the former occupying 
at least the hinder half of the back and the latter somewhat shorter, 
arising more posteriorly; caudal fin forked. Scales ornamented 
with reticulating rugae or pittings, and covering only the anterior 
half of the trunk in advance of the median fins, though some¬ 
times partly extended over the middle of the flank of the caudal 
region. 
Microdon elegans, Agassiz. 
1833-44. Microdon elegans, L. Agassiz, Poiss. Foss. vol. ii. pt. i. p. 16, 
pt. ii. p. 205, pi. lxix. b. 
(?)1839. Gyrodus Iciticciuda, G. von Munster, Neues Jalirb. p. 679. 
1843. Microdon notabilis, G. von Miinster, Beitr. Petrefakt. pt. vi. 
p. 55, pi. ii. fig. 2. [Mandibular dentition; Palaeontological 
Museum, Munich.] 
1851. Pycnodus elegans, A. Wagner, Abli. k. bay. Akad. Wiss., math.- 
phys. Cl. vol. vi. pp. 33, 36. 
1851. Pycnodus notabilis, A. Wagner, ibid. vol. vi. pp. 37, 57, pi. iii. 
fig. 3. 
1862. Microdon elegans, A. Wagner, ibid. vol. ix. p. 343. 
(?) 1862. Microdon nanus, A. Wagner, ibid. vol. ix. p. 344. [Immature 
fish; Palaeontological Museum, Munich.] 
1887. Microdon elegans, K. A. von Zittel, Handb. Pakeont. vol. iii. 
p. 246, fig. 259. 
Type. Nearly complete fish ; Palaeontological Museum, Munich. 
The type species, attaining a length of about 0*3. Maximum 
depth of trunk equalling about four-fifths of the length of the head 
and trunk without caudal fin; head with opercular apparatus 
occupying slightly more than one-quarter of the total length of the 
fish. Splenial teeth closely arranged; those of the principal series 
somewhat broader than long and truncated at each end ; innermost 
teeth and those of the inner flanking series about equal in size, very 
small and sometimes longer than broad ; outermost teeth irregularly 
quadrate, longer than broad, about half as large as the principal 
