STRUCTURE AND FORMULA. OF FIN-RAYS. 55 



cular tail of the fish. Tlie trunk- vertebras of a fish are 

 divisible into those which have free plenrapophyses, called 

 "abdominal vertebrae," and those without, and which 

 terminate below by narrow haemal arches and long 

 spines, called "caudal vertebrae." These haemal arches 

 are formed by difterent parts in different fishes; com- 

 monly by the bent-down and terminally confluent para- 

 pophyses (Fig. 10), I, />, cod ; sometimes, as in the tunny 

 (ib.) Ill, by parapophyses, />, lengthened out by pleurapo- 

 physes, ^Z; sometimes, as in lepidosteus (/&.), II, by pleu- 

 rapophyses, pi; but never, as in air-breathing vertebrates 

 (^6.), Y, by ossified haemapophyses, h, lis. These elements, 

 in the first vertebra of the trunk of a fish, are indeed 

 ossified, and form the long and slender bone called '"'clav- 

 icle," 58 (Fig. 9), usually attached to the inner side of the 

 scapular arch. The haemapophyses of, probably, the last 

 abdominal vertebra, called " ischia," No. 63, are detached 

 from the rest of their segment, and are either loosely 

 suspended in the flesh, beneath or near it, as in the fishes 

 called " abdominal ;" or they are advanced, much elon- 

 gated, and attached to the scapular arch, as in the fishes 

 called " thoracic" (Fig. 9) ; or they are more advanced, 

 shortened, and similarly attached, as in the fishes called 

 "jugular;" or they are wholly wanting, as in the fishes 

 called " apodal." The fins called " ventral," Y, supported 

 by the pelvic haemapophyses, indicate by their position 

 the orders of fishes called " abdominal," " thoracic," and 

 "jugular," by Linnaeus. 



The only proper fins in pairs are the " pectoral," P, 

 answering to the fore-limbs of quadrupeds, and the "ven- i 

 tral," Y, answering to the hind-limbs. The rest of the 

 fins are single and median in position, and are due to 

 folds of the skin, in which certain dermal bones are 



