A CIPENSEROIDEI 



315 



Sub-Order 2. ACIPENSEROIDEI. 



According to the researches of Traquair [451] and A. S. 

 Woodward [505], the families in this group form a degenerating 

 series starting from a type similar to the Palaeoniscids ; they reach 

 the highest degree of differentiation in the Acipenseridae. 



The body remains fusiform with a typical bifurcated hetero- 

 cercal tail. The dorsal and anal fins are moderately short. The 

 orbits are far forward, near the olfactory capsules, but the snout 

 becomes greatly prolonged beyond them and the mouth. 



This rostrum is borne by a prolongation of the median ethmoid 

 cartilage (Fig. 293). The cartilaginous cranium is, indeed, in living 

 forms massively developed, and extends far back, passing gradually 



Left-side view of tlie skull ami vertebral column of Aeipenser. The branchial arches have 

 been removed. (From Owen, Anatomy of Vertebrates, by permission of Messrs. Longmans and 

 Co.) a, notochordal sheath ; b and c', basiventral ; c, pleural rib ; il, basidorsal ; il', inter- 

 dorsal ; e, neural spine; f, cartilage cranium; /, anterior neural arches fused on behind; 

 9> &> Q'"> parasphenoid ; g", articulation of hyomandibular ; 3"", vomerine plate ; ', orbit ; /,', 

 nasal capsule; m, hyomandibular ; o, maxilla; p, palatine; ., 'jugal'; 20, pterygoid carti- 

 lage ; 2(1, hyon:andibular cartilage ; 28, symplectic ; 32, dentary. 



into the vertebral column. Some half-dozen sclerotomes are more or 

 less completely fused with the back of the skull behind the vagus 

 (Sewertzoff [406]) ; there is no break between the skull and the verte- 

 bral column, and the notochord passes uninterruptedly forwards to 

 the pituitary region (Fig. 293). 



The whole cranium is supported below by an immense para- 

 sphenoid, and the vomers are pushed forwards to below the 

 rostrum. The palato-quadrate arches meet in the middle line, which 

 is exceptional among Teleostomes, and lose their articular connec- 

 tion with the ethmoid and sphenoid regions of the skull. The jaws 

 are weakened, become freed from the skull, and lose the premaxilla. 

 The large hyomandibular is only loosely connected with the jaws 

 by means of ligaments and a large symplectic. The five branchial 

 arches are well developed. The frequent presence of a hyoid 

 demibranch (opercular gill), of an open spiracular cleft, and of a 

 spiracular pseudobranch, indicates primitive structure. The gill- 



