58 WE AT/DEN AND PURBECK FOSSIL FISHES. 



The axial skeleton of the trunk is nearly complete, comprising slightly more than 

 30 arches, of which about a third are caudal. Traces of the laminar expansions 

 are seen both on the neural and the haemal spines. A few of the dorsal tin- 

 supports (d.) occur just behind the abdominal squamation, but no other remains of 

 the fins are preserved. The scales are in slightly more than 20 regular transverse 

 series, and exhibit both their internal and their external characters. The peg-and- 

 socket articulation is very deep on all the scales, but the internal riblet is largest 

 on the scales of the lower part of the abdominal region (fig. 2 c). Each scale is 

 deeply overlapped in front, and its exposed portion is covered with large tubercles 

 of ganoine (fig. 2 I>). Each dorsal ridge-scale (fig. 2 a) is also provided along the 

 middle line with a row of eight smooth conical denticles, which are nearly equal in 

 size. The next row of scales below the dorsal ridge is traversed as usual by a 

 slime-canal, and the lateral line is traceable along the middle of the flank. The 

 number of scales in each transverse series is uncertain, but there seems to be about 

 nine above the lateral line. 



Horizon and Locality. — Middle Purbeck Beds: Swanage, Dorset. 



Addendum. — Detached jaws of Mesodon and Eomesodon are not uncommon in 

 the Purbeck Beds, but it is still not possible to name them specifically. The left 

 splenial dentition shown in PI. XIII, fig. 3, is remarkable for the crowding of its 

 principal teeth and the irregularity of its inner and outer small teeth. Another 

 left splenial dentition (PI. XIII, fig. 4) may probably be referred to Mesudon rather 

 than to Mil- rod on on account of its well-developed inner row of small teeth, though 

 this feature is not absolutely distinctive. 



Genus MICRODON, Agassi/.. 

 Mierodon, L. A^assiz, Poiss. Foss., vol. ii, pt. i, 1833, p, 16, and pt. ii, 1844, p. 204. 



Generic Characters. — Trunk deeply fusiform or discoidal, with short slender 

 caudal pedicle. Cranial shield without supratemporal vacuities. Head and 

 opercular bones ornamented with reticulating ruga? and pittings ; two chisel- 

 shaped teeth in each premaxilla and dentary ; tritoral teeth smooth, sometimes 

 feebly indented in the lateral series ; vomerine teeth in five longitudinal series, but 

 the inner lateral pairs regular!) 7 alternating with the widely spaced median teeth ; 

 splenial teeth in four series, the innermost being relatively small, 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, articulated, 

 and much divided distally. Pelvic fins present ; dorsal and anal fins 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 reticulated 



