478 EDWIN S. GOODRICH. 



is important to notice that, in their histological characters, 

 the segments of the lepidotrichia closely resemble the body- 

 scales, as has often been mentioned by previons writers. 

 The scales, like the segments of the dermal ray, are joined 

 together by ligamentons connective tissue. A transverse 

 section through both scale and ray exhibits the same bony 

 and ganoine layers. 



A study of the development of the dermal rays of Lepi- 

 dosteus shows, firstly, that the actinotrichia are peculiarly 

 well developed at the edge of the fins (fig. 19), where they 

 are thicker, longer, and more extensively distributed than in 

 most Teleosts ; secondly, that the segments of the definitive 

 ray develop in exactly the same way as do the lepidotrichia 

 of Salmo. They appear first as thickenings of the basement 

 membrane, which become separated off, and rapidly grow in 

 thickness as they sink inwards. Soon they acquire bone- 

 cells and Haversian canals, and lastly a covering of ganoine 

 is deposited on the outer surface. The structure and 

 development of the body-scales of Lepidosteus have been 

 studied of late years by Hertwig (17) and Nickerson (26) ; 

 and it has been shown that a number of small denticles of 

 often transient nature are developed on the outer surface of 

 the scale. Quite similar denticles are also found on the 

 young segments of growing lepidotrichia. Text-fig. 2, p. 485, 

 illustrates the relations of the dermal skeleton of the fins of 

 Lepidosteus. 



Polypterus, 



The general structure and relations of the jointed dermal 

 rays of this interesting fish have already ])een described by 

 Hertwig (17), Bridge (4), and Boulenger (3). Although highly 

 specialised in some respects, they conform on the whole to 

 the ordinary Teleostean type of lepidotrichia. The anal fin 

 is remarkable in that the dermal rays have preserved the 

 more simple and primitive relation to the endo-skeletal sup- 

 ports. The lepidotrichia here have pointed terminal segments 



