STRUCTURE AND AFFINITIES OF THE PLATYSOMIDA. 377 
vertebral bodies either in this or in any other genus of the family to which it 
belongs. It is also incorrect to represent the rays of the dorsal and anal fins 
as equal in number to their supporting interspinous bones and articulated to 
their extremities ; the real state of matters being, that as in the Paleoniscide 
their rays are more numerous than the ossicles which carry them, whose ex- 
tremities they also overlap. 
On the other hand it is abundantly and clearly demonstrable that Acassiz 
was perfectly correct in representing the dorsal and anal fins as borne by two 
sets of interspinous bones, of which the proximal set (interapophysaires) 
extends right on to the occiput ; while the distal set, immediately supporting 
the rays, is limited in extent to the length of the fin. I should think it also 
extremely probable that two sets of interspinous bones were also present in the 
case of the anal fin, though in the specimens I have examined the evidence is 
not quite so clear, but I am inclined to doubt the existence of the specially 
large one which in AGassiz’s figure commences the series immediately behind 
the abdominal space. Regarding the presence or absence of ribs I regret that 
I am unable to offer any original observations. 
CONCLUSION. 
From the researches recorded in the preceding pages, it will now be 
abundantly clear that the genera treated of forms a connected series whose 
leading structural features may be summed up as follows :— 
The body, deeply fusiform in Hurynotus, or ovoid as in Benedenius, becomes 
very deep and laterally flattened in most of the genera, and often rhombic in 
its contour. The tail is completely heterocercal and accipenseroid in aspect ; 
the deeply cleft caudal fin is strongly inequilobate in some, less so in others ; 
the dorsal margin of the caudal body prolongation is set with a line of imbri- 
cating V scales, its sides clothed with small scales of an acutely lozenge-shaped 
figure. The scales of the body are arranged in dorso-ventral bands, which in 
the more deeply bodied forms become less oblique and more vertical in their 
direction. The scales are articulated by strong pointed processes of the upper 
margin, and in all save Hurynotus (Benedenius ?) the vertical rib or keel of the 
attached surface is coincident with or close to the anterior margin. The dorsal 
fin is long, and, commencing at or behind the middle of the back extends to 
the tail pedicle, while the anal shows every gradation from the short-based 
triangular shape seen in Hurynotus and Benedenius to one closely simulating 
the dorsal in form and extent (Platysomus). The paired fins are largely de- 
veloped in Hurynotus, but they seem to become relatively smaller as the body 
deepens ; this is especially the case with the ventrals, which are rarely seen in 
Platysomus, and have not yet been detected in Cheirodus. ‘The fins are pro- 
