STRUCTURE AND AFFINITIES OF THE PLATYSOMIDA, oll 
horizon of the Millstone grit. P. declivus of Acassiz having turned out to be 
a Eurynotus, the genus must at least be struck out of the list of fishes of the 
Scottish Calciferous Sandstone series, remarkable as these strata for the num- 
ber and variety of their Palwoniscide. 
Structure.—The body is deep, and the general form usually more or less 
rhombic, owing to the pointed snout, and the angulation of the dorsal and 
ventral margins. In P. tenwistriatus the back is, however, gibbously rounded ; 
in P. parvulus (Pl. VI. fig. 5), it forms a high peak in front of the dorsal fin ; 
more commonly, as in P. striatus (Pl. VI. fig. 1), it forms an obtuse angle at 
the commencement of that fin ; the ventral line being more horizontal as far as 
the commencement of the anal, where, forming an obtuse angle, it slopes rapidly 
upwards to the tail pedicle. 
The scales of the body are arranged in nearly vertical bands, which show, 
however, a slight backward and downward obliquity, which increases towards 
the tail; in P. striatus these bands, along the origin of the anal fin, turn for- 
wards at an obtuse angle. In shape the scales (figs. 2, 3, 4) are high and nar- 
row on the flank, but as usual they become proportionally lower towards the 
dorsal, ventral, and caudal aspects. The articular spine is well marked, but 
the internal rib, close to the anterior margin, as in all the genera of the family 
save Hurynotus, varies in strength in different species, beng in some (P. par- 
culus, Forsteri) nearly obsolete. There is a line of small azygous scales, furnished 
in P. parvulus with recurved points, along the dorsal and ventral margins in 
front of the dorsal and anal fins, and along the upper margin of the caudal 
body prolongation they are large and V-shaped, while the sides of the same 
part are clothed with the usual small acutely lozenge-shaped scales, arranged 
in oblique rows, whose direction is from above downwards and forwards. The 
external sculpture of the scales is very characteristic of the genus, and consists 
of fine strize or ridges more or less vertical to the long axis of the body, con- 
sequently tending, on the flank scales, to become parallel with the anterior and 
posterior margins, numerous specific variations occurring, however, in their 
relative coarseness or fineness, straightness or wavyness, while in the marginal 
scales of P. striatus some amount of granulation is also observable. 
The tail pedicle is slender, the caudal fin deeply cleft and heterocercal, 
though not very inequilobate, and the prolongation of the body along the upper 
lobe is comparatively weak. The dorsal fin, commencing at or near the middle 
of the back, is more or less acuminate in front, and extends fringe-like to the 
commencement of the tail pedicle ; the anal is similar in shape and in relative 
position on the ventral aspect of the fish, though its base does not extend quite 
so far forwards. No ventral fins were observed by Acassiz, though he intro- 
duced them hypothetically into his restored figure of the genus (‘‘ Poissons foss.” 
Atlas, vol. ii. Pl. D), and their existence was denied altogether by Professor 
VOL. XXIX. PART I, dC 
