214 ZOOLOGY SECT, 



are always placed immediately beneath the peritoneum. There 

 may be one or more sets of intermuscular bones, attached either 

 to the neural arch (epincumls), to the centrum (epicentrals), or to 

 the ribs (epipleurals). The posterior end of the vertebral column 

 is turned up in the Sturgeons, Lepidosteus, and Amia, resulting 

 in a heterocercal tail-fin : in Amia, however, the fin-rays are so 

 disposed that the fin appears almost symmetrical. Among 

 Teleostei the tail-fin is rarely as obviously unsymmetrical as in 

 the Trout : usually in the adult the development of the large, fan- 

 shaped, posterior haemal arches completely hides the upturned end 

 of the notochord, and in some cases the spinal column ends simply 

 in a somewhat compressed centrum around which the fin-rays 



FIG. 837. Skull of Sturgeon, with the membrane bones removed. <i, pharyngo-branchial : 

 AF, antorbital process ; A R. articular ; b. epibranchial ; c. cerato-branchial ; 6', notochord ; Cop. 

 basi-branchials ; d, hypobranchial ; De. dentary ; GK, auditory capsule ; JED/, hyomandibular ; 

 hy. hyoid cornu ; Hi. inter-hyal ; Md. mandible ; No,, nasal capsule ; Gb, neural arches ; 

 PF, post-orbital process ; PQ. palato-quadrate ; Ps. Ps'. Ps". parasphenoid ; Psp. neural spines ; 

 Qu. quadrate ; R. rostrum ; RL ribs ; Sp. N. foramina for spinal nerves ; S>/. symplectic ; WS. 

 vertebral column; n, vagus foramen; / V, branchial arches. (From Wiedersheim's Com- 

 varative Anatomy.) 



are symmetrically disposed ; such truly symmetrical tail-fins are 

 called diphycercal. 



In the structure of the skull the Chondrostei make the nearest 

 approach to Elasmobranchs. The cranium (Fig. 837) is an un- 

 divided mass of cartilage with a few isolated cartilage bones. 

 The roofing membrane bones lie in the dermis, so as to be practi- 

 cally superficial, and behind pass insensibly into the scutes 

 covering the trunk : the fact that these bones (parietals, frontals, 

 &c.) are exoskeletal structures is here perfectly obvious. The 

 same is the case in Polypterus (Fig. 838), in which, however, the 

 cartilage bones are better developed. In Lepidosteus and Amia, 

 and especially the latter, the skull resembles that of the Trout in 

 all essential respects, the main differences consisting in the 

 absence of certain bones, such as the supra-occipital, and in the 

 presence of additional membrane bones. Among Teleostei it is 

 only in the Physostomi that the membrane bones remain separable 



