38 No Vertebra in Old Red Sandstone Fishes. 



Bat there appears no trace of them, and even in the specimens 

 of the Coccosteus referred to, we see distinctly that the apo- 

 physes rested upon an undivided and continuous axis. Now 

 this incomplete development of the osseous system of the 

 trunk is found among all embryos, and, in particular, among 

 those of fishes ; it is likewise found in the last gradations of 

 the class of fishes, among the Cyclostomes. This series of ver- 

 tebral bodies, which follow each other throughout the whole 

 length of the trunk of vertebrates, is replaced in the inferior 

 forms of this department, and also in embryos, by a cylindri- 

 cal cord of a gelatinous consistence, which is called the dor- 

 sal cord. It is not till some time after the appearance of the 

 cord, that the apophyses and the bodies of the vertebrae are 

 developed in embryo. In the Branchiostoma {Amphyoxus), 

 there is only one cord, without any other piece of skeleton, 

 as among embryos not far advanced. It is among the 

 Cyclostomes that the formation of apophyses commences, and 

 among the Plagiostomes that of the bodies of the vertebrae. 

 In this respect the fishes of the old red sandstone have re- 

 mained at a degree of development altogether embryonic ; for 

 they have a cord and apophyses, but they have no vertebral 

 bodies. 



This disposition of the osseous system of the trunk, almost 

 necessarily determines that of another, — the incomplete de- 

 velopment of the cranium. We find, indeed, in the fishes of 

 the old red sandstone, the exterior bones of the cranium well 

 formed ; the jaws, the thoracic girdle, the opercular and 

 branchiostegous bones, and those of the upper part of the 

 cranium, are well developed, strong, and evidently of a bony 

 structure ; but all that I have observed respecting the forma- 

 tion of the head, leads me to think that the internal case of 

 the cranium, that which immediately surrounded the brain, 

 was not consolidated, but rather cartilaginous. We likewise 

 find this structure in embryos, where the protective plates 

 which cover the top and base of the cranium are developed 

 in an insulated manner, while the cranial case is still cartila- 

 ginous. The same conformation appears in the sturgeon, 

 the osteology of which I have described in my Becherches sur 

 les Poissona Fossiles (vol. ii., 2d part, p. 277); and it is, in fact, 



