49^ FISHES CHAP. 



Aniiidae, 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 

 arcualia, 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- 

 cereal, 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. 



