82 PISCES. 



Sub-Order 1. Chondrostei. 



Palaeontology seems to demonstrate that the modern stur- 

 geons and paddle-fishes are the highly specialized (in some 

 respects degenerate) representatives of the most primitive group 

 of Actinopterygians to which reference has been made above. 

 The skeletal characters they exhibit, of sub-ordinal value, are 

 (i.) the presence of well-developed baseosts in the pelvic fins, 

 (ii.) the non-correlation of the dermal rays of the dorsal and 

 anal fins with their supporting cartilages which form a regular 

 series, and (iii.) the presence of infraclavicular plates. The 

 heterocercal tail is also of prime significance ; but the de- 

 generation of the teeth and scales and most of the minor 

 modifications in the head appear to be characters of secondary 

 importance. These modern fishes represent only four genera 

 and two families, whereas at least five extinct families can be 

 recognized, the earliest with very numerous and very varied 

 members. 



The earliest Chondrostei exhibit the most nearly complete 

 and normal development of the external head-bones and 

 opercular bones, while they also are characterized by the most 

 regular squamation. They are commonly grouped together in 

 the family of Palaeoniscidae, but exhibit much more variety 

 than is usual in families among modern fishes. They range 

 from the Lower Devonian to the uppermost Jurassic (Purbeck 

 Beds) inclusive, but are most abundant in the late Palaeozoic 

 rocks, rare in Mesozoic formations. No single genus is suf- 

 ficiently well known to exhibit all its osteological characters, 

 and the description of the skeleton of the Palseoniscidae must 

 therefore be of a general nature. 



The Palaeoniscid chondrocranium is unsatisfactorily known, 

 but the membrane-bones are always conspicuous in well- 

 preserved specimens. The cranial roof is provided with a 

 continuous shield, of which a small pair of parietals (fig. 61, p.) 

 and the flanking squamosal plates (q.) form the hinder portion, 

 while a large pair of elongated frontals (/!), with or without 

 a separate pair of lateral plates, constitutes the middle por- 

 tion ; a large rostral plate (e.) completing the shield anteriorly, 



