42 FISH OF THE OLD RED SANDSTONE. 



parallel with the curved margin of the scale^ and of course 

 converging towards the middle, they are crossed by fine radi- 

 ating strise ; scales of the sides rhomboidal, nearly square and 

 juxtaposed, each scale articulated to the preceding and superior 

 one by a narrow smooth border on the superior and anterior 

 sides, extending into an angular articulating process at the 

 anterior superior angle (as in Osteolepis, &c.) ; the quadrate 

 exposed portion has a small central point, round which the 

 little rough ridges gyrate diagonally ; bones of the head scul- 

 ptured with small vermicular ridges and granules. 



Length about 11 inches, greatest width of body 2 inches; 

 length of exposed portion of scales about 2 lines. 



The different shape of the scales, and the wide, short, rapidly 

 tapering figure easily distinguish this from the last. 



Not uncommon in the old red schists of Orkney. 



{Col. University of Cambridge.) 



Holoptychius (Ag.). 



Although so large a number of species of this genus are now 

 published, I believe the form of the tail, and number and position 

 of the vertical fins remain unknown. I have recently ascer- 

 tained that the tail is short and perfectly heterocercal, and that 

 one dorsal and one anal fin, nearly equal, but the dorsal largest, 

 exist close to the caudal fin, and opposite each other ; the ven- 

 trals are broad, placed behind the middle of the fish near the 

 anal fin. 



Holoptychius princeps (M^Coy). 



Sp. Char. Scales subquadrate, slightly convex, each about 3 inches 

 wide and 2~ inches long ; anterior concealed margin widest, 

 convex; posterior, exposed portion about one-third narrower, 

 subtruncate rounded, sides slightly concave ; whole of the ex- 

 posed surface closely covered with irregular tubercles about 

 half a line in diameter and half their diameter apart ; most of 

 the tubercles are a little elongated, but in irregular directions, 

 and towards the anterior margin a few of them are generally 

 confluent at their bases, forming short, irregularly twisted, 

 strongly tuberculated ridges ; concealed anterior portion and 

 interval between the tubercles minutely porous. 



This species far exceeds the H. nobilissimus (Ag.) or H. gigan- 

 teus (Ag.) in size ; it is easily distinguished by its entirely tuber- 

 culated surface. A fragment of this species is well figured 

 (without a name) in Murchison^s ' Silurian System,' pi. 2 bis, 

 fig. 3. 



Old red conglomerate of Scat Craig. 



{Col. University of Cambridge.) 



