496 FISHES CHAP. 
Amiidae, may possibly occupy this position, but others, such as 
the Pycnodonts, for example, seem to be highly specialised and 
terminal offshoots which have left no descendants. Of the more 
generalised features which different Holostei retain, mention may 
be made of the prevalence of rhombic scales which, like the 
dermal cranial bones, are generally invested by a variously 
ornamented coat of ganoin; the presence of fulcra, cheek-plates, 
post- or sub-orbital ossicles, and of a complex lower jaw, which 
includes dentigerous splenials; and the abdominal position of 
the pelvic fins. On the other hand, indication of advancing 
specialisation in the Teleostean direction are to be noted in the 
numerical agreement between the dermal fin-rays of the median 
fins and their supporting radialia, and in the character of the 
vertebral column. Some Holostei, especially the earlier forms, 
are acentrous, but between this primitive condition and the 
possession of well-ossified centra, associated with equally bony 
arcuaha, almost every gradation is to be found. The chondro- 
cranium is more or less completely replaced by cartilage bones 
corresponding to those generally present in Teleosts, while the 
palato-pterygoid cartilages, likewise modified by the growth of 
cartilage bones, separately articulate with the lateral ethmoid 
regions instead of meeting in a ventral symphysis beneath the 
basis cranii. With rare exceptions (e.g. certain Pycnodonts) the 
opercular skeleton is complete, and includes branchiostegal rays ; 
and although a single gular plate is often present, it may be 
absent in entire families. Like so many other structures, the tail 
is in a transitional state: really heterocercal, but incipiently homo- 
cercal, it may be described as semi-heterocercal. Infra-clavicular 
plates no longer form part of the secondary pectoral girdle, their 
place being taken by cleithra which, as in most Teleosts, meet 
in a ventral symphysis. 
Indications of transition are not wanting in the squamation 
in certain families, and may be seen in the partial or complete 
replacement of the rhombic type by thin, imbricated, cycloid 
scales. Lastly, the soft parts of the two surviving genera are not 
without features of similar significance. A multivalvular conus 
arteriosus, it is true, is still retained, but the spiral valve is 
vestigial, the spiracles are closed, and in the female of one genus | 
(Lepidosteus) the gonoducts are peritoneal tubes, continuous, as 
in most Teleosts, with the investments of the ovaries. 
