412 THE ANIMAL KINGDOM. 



genus Diodon with its immediate allies has projecting spines with bare 

 skin between. Bony shields of great size are found in many living Siluroidei, 

 but especially in the extinct Ganoid Placodermi (Pterichthys, Cephalaspis] 

 from Devonian and Carboniferous strata, but they are restricted to the 

 fore-part of the body 



The dermal exoskeleton appears to have given origin to the investing 

 bones of the skull and shoulder-girdle, which adapt themselves more and 

 more to the underlying cartilage, and eventually replace it in part in the 

 former. The fin-rays supporting the azygos fins and the edges of the 

 paired fins have a similar origin. Each ray primitively consists of a series 

 of pieces, which are paired right and left in the azygos fins, dorsal and 

 ventral in the paired. 



The continuous azygos fin of the embryo persists in the adult of some 

 Fish, e. g. the Teleostean Eel, Sole, Cod (Anguilla, Pleuronectes, Gadus] 

 probably by reversion (Balfour). It is usually broken up into one or more 

 dorsals, a caudal, and one or more anals. The fin-rays are horny in 

 Elasmobranchii ; cartilaginous throughout in chondrostean Ganoidei and 

 the Dipnoi, but with a thin shell of bone formed in the perichondrium in 

 the latter ; whilst in Teleostei and bony Ganoidei they are ossified. In the 

 latter case they may be soft and consist either of a series of joints or be 

 branched dichotomously ; or entire, i.e. spinose, and of one piece throughout. 

 In some Elasmobranchii and in Polypterus the sections of the dorsal fin are 

 carried each by a strong spine (ichthyodorulite) ; in a few Teleostei its 

 posterior lobe contains only horny fin-rays, is soft and fatty, forming a small 

 adipose fin (Salmonidae, some Siluroidei, &c.). The fin-rays of the ventral 

 portion of the caudal fin are carried by haemal arches ; but those of its 

 dorsal portion, of the dorsal and anal fins by skeletal elements lying in 

 the median fibrous septum dividing the right and left halves of the body 

 (see p. 101). The caudal fin, which is an all-important organ of locomotion, 

 is either heterocercal, homocercal, or diphycercal. It is heterocercal when 

 the vertebral axis is bent dorsally and terminates in a lobe or point, and 

 the caudal ventral lobe is placed at a greater or less distance from its 

 extremity ; homocercal when it is outwardly symmetrical, but in reality 

 consists almost entirely of a caudal ventral lobe, the extension of the 

 vertebral axis beyond the lobe being atrophied ; and diphycercal when it 

 is divided into equal dorsal and ventral lobes by a straight vertebral axis. 

 A heterocercal caudal fin is found in Elasmobranchii, Holocephali, chon- 

 drostean and many extinct Ganoidei ; a homocercal in living bony Ganoidei, 

 and the majority of Teleostei', and a diphycercal in a few Teleostei and 

 the Dipnoi (see p. 97). In some Ganoidei, living, e. g. Acipenser, Lepidosteus, 

 and extinct, a single or double row of small spines known as fulcra extend 

 along the anterior margins of the dorsal or caudal fin, and on the ventral 

 edge of the latter as well in Lepidosteus. 



