976 



CLASS PISCES. 



As an Acipenseroid of uncertain position may also be mentioned 

 the imperfectly known Pholidurus, of the English Chalk — the tail 

 of which presents some resemblance to that of Psephurus. A huge 

 fish from the English Upper Lias, named Gyrosteus, is evidently 

 also an Acipenseroid agreeing with the type genus in the absence 

 of teeth, but having a naked body. It differs also considerably 

 from the type of the next family, and its position must remain 

 uncertain. 



Family Chondrosteid^e. — This family is formed for the recep- 

 tion of the one comparatively small fish constituting the genus 

 Chondrosteus from the English Lower Lias. According to Dr 

 Traquair, although there is no evidence of a long snout (fig. 913), 

 this genus resembled Polyodon in the general form of the fins, and 

 of the nearly naked body ; but in other respects — such as the form 

 of the cranial bones and the absence of teeth — approaches Acifienser. 

 Certain features also appear to indicate distinct affinity with the 



Fig. 913. — Profile view of the head of C kondrostetcs acifienseroides ', from the Lower Lias of 

 Dorsetshire. Reduced, f, Frontal ; j>.f, Postfrontal ; p, Parietal ; sq, Squamosal; s.t, One of 

 the supratemporals ; p.t, Posttemporal ; s.cl, Supraclavicular; ci, Clavicular; i.cl, Infraclavic- 

 ular; op, Opercular; s.op, Subopercular ; br, Branchiostegals; c.h, Ceratohyal; ag, Angular; 

 d, Dentary; mx, Maxilla;/, Jugal ; k.m, Hyomandibular ; s.o, Suborbital. (After Traquair.) 



Palceoniscidce ; and since the latter are evidently allied to the higher 

 bony Ganoids, Mr Woodward suggests that they or allied forms 

 may have given origin to two series, one of which culminated in the 

 Teleosteans, while in the other " the only advance has been in the 

 matter of size, and this accompanied by a certain amount of de- 

 generation, culminating in Acipenseroids proper." 



Family Pal^eoniscid^. — This and the next family constitute the 

 suborder Heterocerci of some writers. In the present family the 

 body is fusiform, and covered with rhomboidal scales ; the vertebral 

 arches are ossified, but there are no bony ribs ; the dorsal fin is 

 single and short ; all the fins have large fulcra at their bases ; the 



