A C1PENSEROIDEI 



3'9 



and reduced in size. The small jaws, toothless in the adult, together 

 with the palato-quadrate arch, form a specialised apparatus connected with 

 the powerful hyomandibular by a large symplectic (Fig. 293). A maxilla 

 and a 'jugal' are present as in Ckondrosteus, also ossifications probably 

 representing the palatine, ectopterygoid, metapterygoid, and quadrate in 

 the expanded cartilage of the palato- quadrate arch. A single bone is 

 supposed to represent the 

 vomers. Only a dentary is 

 developed on Meckel's carti- 

 lage. In old specimens of 

 Acipenser- ossifications appear 

 in the chondrocranium, which 

 probably represent the opis- 

 thotics, prootics, orbito- 

 sphenoids, and two lateral 

 ethmoids. The visceral arches 

 are partly ossified (Parker 

 [321], van Wijhe [494], Col- 

 linge [86]). Supra-, post-, 

 and suborbitals persist, also a 

 large opercular, an inter- 

 opercular, and a subopercular ; 

 the opercular membranes are 

 confluent below. There are 

 no branch iostegals. In Aci- 

 penser, but not in Scaphi- 

 rhyndius, the spiracle persists, 

 with a pseudobranch. A 

 hyoidean hemibranch is 

 present. 



On the trunk are five 

 longitudinal rows of large 

 rhombic plates, and some 

 scattered small irregular 

 spines (Fig. 299). The front 

 edge of the pectoral fin is 

 strengthened by a powerful 

 spine rigidly attached to the 



pst 



FIG. 298. 



Skull of Acipenser sturio, L. ; dorsal view. A dotted 

 small first radial articulating line indicates the lateral line according to Collinge. 

 . ,. a.n, anterior nostril; fr, frontal ; j, junction of post- 



Wlth the shoulder - girdle ; orbital, occipital, and trunk branches of lateral-line 



this <?ninp rpnrpspnrs n mnrVi s y gtem ! P> opercular; pa, parietal; p.n, posterior 



spine repre 1 nostril . poC) postoccipital ; prf, prefrontal ; pst, post- 



enlarged pair of lepidotrichia temporal ; py, postfrontal ; pto, pterotic ; r, rostral 



L. i ii . . , V -L plates ; so, supraorbital ; soc, supraoccipital ; sr>, 



in which the joints have been spiracle; st, ropratemponi 



obliterated. No ganoine is 



found on the exoskeleton ; it is merely replaced by layers of bone, of 

 which the scales and plates are entirely composed. The internal organs 

 are of generalised character (p. 309). The air-bladder is simple (p. 223). 

 Acipenser, L. (Fig. 299) ; freshwater and marine, Europe, Asia, and N. 

 America ; Lower Eocene, England. 



