444 PKOCEEDINaS OP THE GEOLOGICAL SOCIETY. [DeC. 15, 



external ridge, and therefore Kes altogether on the inner side of the 

 middle line. The transverse concavity is least in the middle of the 

 scute and greatest at its posterior edge. 



Antero-posteriorly the ianer surface is very convex in the middle 

 line, its anterior and posterior moieties meeting in a rounded trans- 

 verse ridge, which is nearer the anterior than the posterior margin. 

 That part of the scute which lies behind this transverse boundary is 

 much thicker than that which lies in front of it. None of the 

 numerous wide angulated scutes which I have met with have been 

 less than two inches in transverse diameter. 



Of the thick angulated scutes to which I have referred, I know 

 only the inner faces, and the minimum thickness, as no one of the 

 specimens (which are all natural casts) shows the outer face. Again, 

 the only specimens I have met with have been associated with 

 remains belongiug to the anterior part of the body, such as the 

 scapulae or the ribs. 



A cast of one of these, on the same slab with the impressions of 

 two scapulse, is two inches long in the middle, and has the same 

 width at its widest part, but it is not quite square. The side which 

 I take to be the inner is nearly straight ; and the inner edge, which 

 is 2^ inches long, appears to have been thick, and serrated for sutural 

 union with its fellow. The anterior side measures about If inch 

 in length, and is somewhat broken, so that its proper contour can 

 hardly be made out. The junctions of the internal anterior and 

 posterior edges appear to have been sharply angular, while the 

 antero-external angle was slightly, and the postero-external angle 

 greatly, rounded oJQf. The internal surface is convex from before 

 backwards, concave from side to side. 



I have found scutes very similar to these, but smaller, associated 

 with the impressions of some ribs. The smallest of these was not 

 more than an inch long, and one which had a length of 1^ inch was 

 fully half an inch thick at its postero-external angle. On the other 

 hand, scutes of this kind appear in some cases to have attained a 

 width of nLore than four inches, and a thickness of seven-eighths of 

 an inch. 



The irregular angulated scutes (fig. 3) are pentagonal or rhom- 

 boidal, the ridge by which they are marked externally projecting so 

 far backwards that their posterior margin (h) becomes triangular. 

 One of the largest of these had a length of 1|- inch by a breadth 

 of half an inch, and had a roughly pentagonal form, its anterior 

 edge being slightly convex. Another had a length of 1^ inch by a 

 breadth of -|ths of an inch, and presented only a very small sculp- 

 tured surface close to its anterior margin. In fact, the proportion 

 of the sculptured to the smooth surface was far less in these than 

 in the wide scutes. 



I have compared the parts which have just been described with 

 the scales of Glyptopomus ; and, though there is a certain resem- 

 blance between the latter and the flat scutes, the dermal plates of 

 no fish with which I am acquainted present any similarity to the 

 angulated and thick scutes. On the other hand, any pne acquainted 



