STRUCTURE AND AFFINITIES OF THE PLATYSOMIDA. 381 
tion, Dr WAGNER, after noticing the absence of the characteristic Pycnodont 
“‘ Vorkiefer ” or premandibular bone, states that the teeth have the same con- 
figuration as in Aichmodus.* 
If therefore Tetragonolepis belongs, not to the Pycnodontide, but to the 
Dapediide (Stylodontes, Wagner), it is to my mind also a step in the wrong 
direction to include Platysomus with it in the same family. 
For the Dapediide (including TYetragonolepis) differ most materially from 
Platysomus and its allies in having a few-rayed semiheterocercal Lepidosteoid 
caudal fin, instead of the many-rayed heterocercal and Acipenseroid one of 
Platysomus ; in the manner in which the rays of the dorsal and anal fins corre- 
spond in number to their supporting interspinous bones ; in the presence of long 
ribs, and of well-ossified hemivertebre (though the notochord is also persistent); 
and in the absence of infraclavicular plates. With these obvious differences in 
the structure of the body is associated, as might be expected, an equally striking 
dissimilarity in the osteology of the head, as may be seen by referring to the 
restored figure of the head of Dapedius which I have constructed (Pl. VI. 
fig. 13), after careful study of the large series of specimens in the collections of 
Lord ENNISKILLEN, of the British Museum, and of the Museum of Practical 
Geology.+ Without entering into any detailed description, it may be sufficient 
to point out that the general features here exhibited are not those of the 
Platysomide, but those of the more modern type of Ganoids exemplified in the 
fossil Lepidotus, Semionotus, &c., and in the recent Lepidosteus and Ama; in 
particular, we may note the completely Teleosteoid aspect of the opercular 
apparatus in which the preoperculum does not extend forwards on the cheek, 
and has associated with it an operculum, suboperculum, and interoperculum, 
arranged quite according to the ordinary pattern. The styliform shape of the 
teeth in some Platysomidz, and the deep form of the body in the Dapediide, 
with the shape of the scales in the special genus Tetragonolepis, seem to me to 
be characters of small importance when placed against the differences in general 
structure, which certainly forbid their association in one “ family,” according to 
* “ Miinchener Gelehrte Anzeigen,” 1860. Dr Waaener here uses Quenstzp?’s name Pleurolepis for 
Tetragonolepis of Bronn, and Tetragonolepis for Aichmodus of Egerton, Mchmodus is distinguished 
from Dapedius, De la Beche, only in having the apices of the teeth simple instead of bifid ; but as Sir 
Puitip Grey-Ecerton has himself pointed out, both forms of tooth may occur in the same specimen, 
and the name 4chmodus is therefore not maintainable. As to the use of “ Plewrolepis,” its priority 
over Tetragonolepis cannot be maintained. It is true that QuENstepT first pointed out that Tetra- 
gonolepis semicinctus, Bronn, was generically distinct from the other species added by Acassiz to the 
same genus, but surely, instead of inventing a new name for the first, and passing Tetragonolepis on to 
the others, he ought to have preserved the original generic name for the original type. 
t The restored figure of the head of “Hchmodus,” given by Professor Youne in his paper “ On the 
Affinities of Platysomus,” is incorrect in at least one important particular, namely, in representing the 
parietals as pushed outwards to a position behind the squamosals by an intruding compound “ supra- 
occipital.” The plate, which he has lettered as “ post-frontal,” seems to me to be only a member of the 
circumorbital ring. 
