PLACOGANOIDEI. 139 



E. Biicldandi, are found in the eocene of Bagshot and Brack- 

 lesham ; and one species (E. helveticus) is from the mollasse of 

 Switzerland. 



Genus Elasmodus, Egerton. — Each upper maxillary has 

 three dental columns, but the dentine is confluent, ''being- 

 rolled round like a scroll on the substance of the bone, one 

 edge forming the margin of the tooth, the other buried deep 

 in its centre."* The premaxillary has a thin incurved scal- 

 priform tooth, rounded at the cutting edge, of a lamellate 

 structure, with a columnar arrangement of the plates, which 

 are juxtaposed. This genus is exclusively represented by 

 species = E. Hunteri — from the London clay of Sheppy. 



Order III.— GANOLDEI. 



Char. — Endo-skeleton in some osseous, in some cartilaginous, 

 in some partly osseous and partly cartilaginous ; exo- 

 skeleton formed by enamelled bones ; fins usually with 

 a strong spine for the first ray. 



Sub-order 1.— PLACOGANOIDEI. 



Char. — Endo-skeleton cartilaginous, or retaining the noto- 

 chord ; head and more or less of the trunk protected by 

 large ganoid, often reticulated, and suturally united, 

 plates ; heterocercal. 



The last term signifies a form and structure of tail illus- 

 trated by fig. 58, and to be seen in the sharks, dog-fishes, and 

 sturgeons of the present day : it results from a prolongation 

 of the vertebral column, n, into the upper lobe dn, producing 

 an imsymmetrical form of the caudal fin, which is contrasted 

 with the symmetrical form of the same fin presented by most 

 fishes of the present day, and illustrated by the Leptolepis 

 sprattiformis (fig. 73), and by the Semiophorus (fig. 76), in 



* Egerton, Tree. Geo!. Soc, May 12, 1847. 



