160 SUB-CLASS GANOIDEI. 



agree with those zoologists who wish to unite the Ganoids and 

 Teleosteans into a single group, distinct from other piscine orders. 

 In this opinion we are in company with two of the greatest ana- 

 tomists of the last century, J. Miiller and F. M. Balfour. The 

 latter has expressed his view in words, with which we are in 

 entire agreement and which we quote here, because they appear 

 to express in the most judicial form the state of the question. 

 He says, " We do not recommend such an arrangement (union 

 of the Ganoids and Teleosteans) which in view of the great pre- 

 ponderance of the Teleostei amongst living fishes would be highly 

 inconvenient, but the step from Amia to the Teleostei is certainly 

 not so great as that from the Chondrostei to Amia, and is un- 

 doubtedly less than that from the Selachii to the Holocephali." 



The scales present some variation in arrangement and struc- 

 ture. In the living Chondrostei they may be almost absent on 

 the body as in Polyodon, or arranged in rows as in Acipenser ; 

 not, however, forming a continuous cuirass except in the caudal 

 region. They frequently carry bony spines, which are without 

 any enamel cap. In some extinct Chondrostei they form a con- 

 tinuous cuirass, and have often the rhombic form typical of the 

 order. In Crossopterygii they form a continuous armour and 

 are either rhombic or cycloidal ; in the living forms it can be 

 shown that they are coated externally by ganoin and that in 

 some cases they carry spines which consist of cones of dentine 

 capped with enamel. In Lepidostei there is also a continuous 

 armour of rhombic or cycloid scales, and in the living Lepidosteus 

 it has been shown that these scales are coated with ganoin 

 (yavos sheen) and may, especially in the young state, carry one 

 or a number of small spines having exactly the structure of 

 the spine of a placoid scale. 



In Amia the scales, which form a complete armour, are 

 Teleostean in character and consist of bony plates without ganoin. 

 Moreover it has been shown from a study of living forms that 

 the scales save for the toothlike projections which occur in Poly- 

 pterus and Lepidosteus are purely mesodermal products, and that 

 the ganoin which was formerly thought to be enamel is really 

 of the nature of vitrodentin and is formed by the scleroblasts * 

 of the dermis. 



* Scleroblasts are cells which secrete a hard skeletal substance. 



