1866.] YOUNG CAEBONIFEROUS GLYPTODIPTERINES. 601 



No other portion of the skeleton has as yet been recognized. 



Comparison of Williamson's sections of the scales of a Carboniferous 

 Holoptychius (loc. cit. fig. 24) with those in the Poiss. Foss. clu Yieux 

 Gres Rouge, further confirms, by the difference of minute structure, 

 the generic distinctness of the fishes from the two series of rocks. 

 Further discussion of these microscopic characters is reserved for 

 another occasion ; but the vertical section just mentioned, in Wil- 

 liamson's paper, is interesting in this, that while the description 

 corresponds with that of the under surface of Bliizodus-s>(idlQS, the 

 form and structure of the upper surface would agree well with that 

 of a tuberculated scale on which little or no ganoin was developed, 

 such a scale in fact as I believe that of Ehizodus to have been. 

 But the point requires further investigation. 



HoLOPTYCHius, Agassiz (excl. BTiizodus)^. 



The characters of this genus are summed up in the memoir 

 already mentioned (p. 5), thus : — 



" Head depressed ; caudal extremity conically tapering, orbits far 

 forward ; gape extends far back. Frontals distinct from one another 

 and from the parietals, which are large and coadapted, though quite 

 distinct. The occiput is covered by three bones, a median and two 

 lateral. There are two principal and a number of lateral jugular 

 plates, and there is no rhomboidal median plate between the two 

 principal jugulars. Some of the teeth are larger than the others, 

 and longitudinally striate at their bases. The paired fins are very 

 acutely lobate, and there are two dorsal fins in the posterior half of 

 the body. The ventrals are situated under the first dorsal, and are 

 succeeded by a single anal. Tail little more than semirhomboidal, 

 the upper half being much less developed than the lower." 



The diagnosis of this genus from Bhizodus by dentitional charac- 

 ters and the difference in sculpture of the scales has already been 

 spoken of. 



Dendeoptychius, Huxley. Fig. 7. 



The bony scales to which this name was given are characterized 

 by the division and subdivision of the grooves which traverse their 

 exposed surface. They are nearly equilateral, with rounded angles. 

 They vary in size on the same individual ; the largest measure about 

 i|-ths in length by 1 inch in breadth ; but the average is f^^-ths by 

 j6^-ths. The overlapped area is covered with fine striations, concen- 

 tric and radial, the latter being most marked over a triangular area 

 whose base is the centre of the anterior margin. The exposed sur- 

 face, slightly smaller than that of the preceding, is sharply bounded by 

 an arcuate line whose convexity looks forward. From this line the 

 sinuous branching grooves run, those from the centre radially, those 

 from the lateral parts more directly backwards ; they are larger 

 and fewer in number on the larger scales; their interspaces are 

 marked with fine anastomosing grooves. The under surface is smooth, 



* The scales described under this name by Williamson belong, as has been 

 already mentioned, to a wholly different genus, Mhizodopsis. 



2s2 



