SKELETON OF LEPIDOSTEUS. 459 



and by the structure of the skull {e.g. typically 5 otic bones, 

 alisphenoids and oi-bitosphenoids, etc.) and of the hyoid arch 

 (a symplectic, two ossifications in the ceratohyal, etc.). This sub- 

 class contains the great majority of living fishes, and includes a 

 large number of orders, of which the first three are : — 



Order 1. Protospondtli, 



Caudal fin abbreviate heterocercal. Vertebral column acentrous, 

 or with centra variously developed*. Prsemaxillaries fixed, 

 attached to frontals and pierced for passage of olfactory nerves ; 

 maxillary free, unsegmented, usually with a supramaxillary. 

 Lower jaw with "supra-angular," large dentigerous pr^articular, 

 and intra-dentaries. Hyoid arch and prseoperculum normally 

 developed, and symplectic firmly attached to quadrate. Only one 

 coracoid ossification, or none ; mesocoracoid bridge cartilaginous ; 

 one or more pectoral radials inserted on metapterygium. 



Principal families : — SemionotidcC, Macrosemiidse, Eugnathidae, 

 Amiida?, Pachycorraidte, Aspidorhynchidse, Pycnodontidse. 



Order 2. Ginglymodi. 



Distinguished from the preceding by the characters of the 

 jaws, suspensorium, and opercles. Maxillary segmented, the first 

 one or two segments attached to prsemaxillary, the rest to ecto- 

 pterygoid ; metaptery golds articulating with transverse facets on 

 wings of parasphenoid ; hyoid arch free from pterygo-quadrate ; 

 prseoperculum small, its anterior end articulating with a condyle 

 on quadrate ; interoperculum large, fixed, connecting hyomandibu- 

 lar with prseoperculum. Yertebral centra solid, opisthoccelous. 



One family, Lepidosteidje. 



Order 3. Halecostomi. 



Caudal fin abbreviate heterocercal. Prsemaxillaries small, 

 loosely attached ; maxillaiy typically with two supramaxillaries ; 

 lower jaw without '• supra- angular " or intra-dentaries. Yertebi-al 

 centra annular or amphicoelous. 



Two families, Pholidophoridje and Oligopleuridse. 



* The various conditions are : — (1) No centra ; bases of arches not expanded. 

 (This is found in all families e.vcept Amiidai and Aspidorlij'nchidaj.) (2) No centra ; 

 bases of arches nvny form laminar expansions over the notochord (some Pycno- 

 donts). (3) Annular or amphica-lous centra of uncertain origin ; no praj- and 

 post-centra (Aspidorhynchidaj, Neorhombolepis). (4) Solid outgrowths of arches 

 may enclose notochord, forming annular centra, with pra'- and post-centra in caudal 

 region. (Some Macrosemiidse, Eugnathida?, and Pachycormidse.) (5) Centra disc- 

 like, formed by ossification of tissues surrounding notochord, enclosing bases of 

 , arches ; praa- and post-centra in caudal region (Amiidse). 



