ERA OF THE OLD RED SANDSTONE 



39 



which seems to have been the sole organ of motion It 

 is very remarkable, that, while the tail establishes this 

 creature among the vertebrata and the fishes, its mouth 

 has been opened vertically, like those of the crustaceans, 

 but which is contrary to the mode of vertebrata generally. 

 This seems a pretty strong mark of the link character of 

 the coccosteus between these two great departments of 

 the animal kingdom. The pterichthys has also strong bony 

 plates over its body, arranged much like those of a tor- 

 toise, and has a long tail ; but its most remarkable feature, 

 and that which has suggested its name, is a pair of long 

 and narrow-wing like appendages attached to the shoul- 

 ders, which the creature is supposed to have erected for 

 its defence when attacked by an enemy. 



The holoptychius is of a flat oval form, furnished with 

 fins, and ending in a long tail ; the whole body covered 

 with strong plates, which overlap each other, and the 

 head forming only a slight rounded projection from the 

 general figure. The specimens in the low r er beds are 

 not above the size of a flounder ; but in the higher strata, 

 to judge by the size of the scales or plates which have 

 been found, the creature attained a comparatively mon- 

 strous size. 



The other fishes of the system — the osteolepis, glypto- 

 lepis, dipterus, &c, are, in general outline, much like 

 fishes still existing, but their organization has, neverthe- 

 less, some striking peculiarities. They have been en- 

 tirely covered with bony scales or plates, enamelled ex- 

 ternally ; their spines are tipped with bone, and, as one 

 striking and unvarying feature, the tail is only finned on the 

 lower side. The internal skeleton, of which no traces 

 have been preserved, is presumed to have been cartilagi- 

 nous. They therefore unite the character of cartilaginous 

 fishes with a character peculiar to themselves, and in 

 which we see pretty clear vestiges of. the pre-existent 

 crustaceous form. 



With regard to the link character of these animals, 

 some curious facts are mentioned. It appears that in the 

 imperfect condition of the vertebral column, and the infe- 

 rior situation of the mouth in the pterichthys, coccosteus, 

 &c, there is an analogy to the form of the dorsal cord 

 find position of the mouth in the embryo of perfect fishes 

 The one-sided form of the tail in the osteolepis, &c, finds 

 i similar analogy in the form of the tail, in the embryo ot 



