LEPIDOSTEI. 177 



embedded cartilage bones much as in the sahnon, and is com- 

 pletely invested by membrane bones. The vomer is double. 

 There is a symplectic in the suspensorum, several bony elements 

 are present in the mandible, and the maxilla is divided trans- 

 versely into many bones. 



The pectoral girdle is as in Teleosteans, with scapula and 

 coracoid ossifications and overlying clavicle. The pectoral fin 

 has one row of basal elements carrying the dermotrichia. The 

 pelvic girdle is absent. 



In Lepidosteus * the scales of the trunk are arranged in oblique 

 rows which pass from above and in front backwards and ventral- 

 wards. The scales of a row are more closely connected with each 

 other than with those of neighbouring rows, in consequence of a 

 peg and socket articulation. Each scale has its anterior and 

 dorsal angle produced into a pro- 

 cess which fits into an excavation 

 under the next dorsal scale of the 

 same row, where it is attached 

 by a ligamentous band. The 

 centre of the scale is bored by 

 one or several canals which pass ^,^ ms ^ *• * ^t, f 



i Pig. 105. — A portion of the armour of 



right through it and transmit Lepidosteus ■ B single soale showing 



° '^ antero-dorsal process which fits below 



blood vessels. The scale con- the next dorsal scale (after O. Hert- 



wig) ; I foramina for blood vessels. 



sists in its deeper portions of 



bone with bone corpuscles and vertically directed fibres, which 

 are prolongations of large scleroblast cells at the surface (so- 

 called odontoblasts, they do not become enclosed) and of a 

 superficial layer of structureless dentine or ganoin, as it was 

 called by Williamson, which is only found on the exposed part 

 of the scale, and was formerly taken for enamel. In some 

 parts of the body (ventral side of head, investing bones of skull 

 and shoulder girdle, the fin scales) the scales carry one or a 

 number of small teeth, which consist of enamel, dentine with 

 fibres, and a pulp-cavity. In the young animal all the scales 

 carry teeth, with the dentine of which the ganoin of the scale 

 is continuous. The scales are developed as plates in the dermis, 



* 0. Hertwig and Klaatsch, op. cit. ; see also W. C. Williamson, On 

 the microscop. structure of scales, etc., of fishes, Phil. Trans., 1849 and 

 1851. Nickei'son, Scales of Lepidosteus, Bull. Mus. Comp. Zoology^ 

 Harvard, 24, 1893. 



Z. II. XT 



