52 WEALDEN AND PURBECK FOSSIL FISHES. 



specimen) each bears a row of four or five small denticles which increase slightly 

 in size backwards. The lowest flank-scales are scarcely deeper than wide, though 

 very irregular in shape. Above these the flank-scales are much deeper than wide, 

 complete to the number of about four in the foremost series, but gradually reduced 

 to one behind. All seem to be more or less irregular, as already described by 

 Hennig in Mesodon macropterus. 1 So far as can be determined from the riblets, 

 all the flank-scales are deep and must have been few in a transverse series. The 

 course of the lateral line is not clear, but the upper slime-canal from the occiput 

 to the origin of the dorsal fin is marked by slight transverse expansions on the 

 successive riblets of deep scales immediately below the dorsal ridge. As shown in 

 the Dorset Museum specimen, each dorsal ridge-scale is armed with a row of five 

 small smooth denticles which increase slightly in size backwards. In the type 

 specimen an irregularly elongate-triangular smooth scale,- with its short anterior 

 base crimped into three or four digitations (PI. XII, fig. 1 d), occurs within the 

 caudal fin and probably represents the last remnant of the squamation of the 

 upper caudal lobe. 



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



2. Mesodon parvus, A. S. Woodward. Plate XII, figs. 3, 4. 



1895. Mesodon macropterus, var. parvus, A. S. Woodward, G-eol. Mag. [4], vol. ii, p. 147, pi. vii, fig. 2. 



Type. — Fish with incomplete head ; British Museum. 



Specific Characters. — A small species, probably attaining a length of about 

 14 cm. Maximum depth of trunk somewhat less than the length of the fish with- 

 out caudal fin ; head with opercular apparatus contained about three times in the 

 same length; back gently rounded, and dorsal fin arising slightly behind the 

 highest point. Teeth frequently indented with a shallow pit ; principal splenial 

 teeth rounded and smooth, about twice as broad as long, flanked externally by two 

 series of smaller teeth, the inner broader than long, the outer nearly round. Dorsal 

 and anal fins equally elevated, the latter with about 26 supports and two-thirds as 

 long as the former, which has about 36 supports. Dorsal and ventral ridge-scales 

 with few comparatively large denticles, those of each scale rapidly increasing in 

 size backwards. 



Description of Specimens. — The type specimen (PI. XII, fig. 3) is a fish scarcely 

 more than 5 cm. in length ; and a second specimen presented to the British 

 Museum by Mr. T. T. Gething, showing the nearly complete head, is of about the 

 same size. Portions of larger fishes, however, from the same formation and 

 locality, probably belong to this species ; one presented to the Museum of Practical 



1 E. Hennig, "Gyrodus uud die Organisation der Pykuoilonten," Palarontographica, vol. liii (1906), 

 p. 172, fig. 8. , 



