230 



ZOOLOGY 



SECT. 



of the notocliord, and ia some the spinal oohiiiin ends simply 

 in a somewhat compressed centrum around which the fin-rays 

 are symmetrically disposed ; such truly symmetrical tail-fins are 

 called dipliyccrcal. 



In the structure of the shuU tlie Chondrostei make the nearest 

 approach to EUismobranchs. The cranium (Fig. 886) is an un- 

 divided mass of cartilage with a few isolated replacing bones. 

 The roofing investing 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. 887), in, which, however the 

 rei:)lacing bones are better developed. In Lepidosteus -and Amia, 



CK 



JKr 



sh]\r 







n" 



ijjp 



^y 



Mi 



Ma 



top 



Fig. SS6. — Skvill of Sturgeon, witli tlie investing bones removed, a, pliaryngo-bi-anchials ; 

 AF, antorbital process ; AR. articular ; h. epibranchial ; c. ceratobranchial ; C, notocliord ; Cop. 

 basibranchials ; 0., hypobranchial ; J)e. dentary ; GK, auditory capsule ; EM. hyomandibular ; 

 liy. hyoid cornu ; Ih. interhyal ; Md. mandible ; I^a. nasal capsule ; Gh. neural arches ; 

 PF, postorbital process ; PQ. palatoqviadrate ; Pi. Ps'. Ps". parasphenoid ; Psp. neural spines ; 

 Qu. quadrate ; R. rostrum ; Jii. ribs ; Sp. N. foramina for spinal nerves ; %. symplectie ; WS, 

 vertebral column ; ii, vagus foramen ; /— F, branchial arches. (From Wiedersheim's Com- 

 parative Anatomy.) 



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 supraoccipital, and in the 

 presence of additional investing bones. Among Teleostei it is 

 only in the Physostomi that the investing bones remain separable 

 from the chondrocranium in the adult; in the remaining orders, 

 e.g. in the Cod, Haddock, or Perch, they become grafted on to the 

 chondrocranium and so closely united with the replacing bones that 

 they can be removed only by pulling the whole skull to pieces ; 

 most of the original cartilage frequently disappears in the adult 

 and the cranium thus becomes a firm bony mass in which no dis- 

 tinction between replacing and investing bones is discernible. 



The varying size of the gape, which is so noticeable a feature in 

 the Teleostomi, depends upon the inclination of the suspensorium : 

 in wide-mouthed Fishes (Fig. 877) the axis of the hyomandibular 



