No. 380.] DEVONIAN PTYCTODONTIDE. 555 
body segments; and that from this condition arose a more or 
less typical spine shaft, thick at one end and pointed at the 
other, with indications that its decurved character was accom- 
panied by a firmer insertion of the proximal end, and the evasion 
of a pointed tip.” The stages of differentiation passed through 
by this genus are clearly depicted by Dr. Dean. We cannot 
well dissent from his view that the segmental structure observed 
in all species of Edestus is evidence of a primitive condition, 
and yet Belemnacanthus, a much earlier form, shows no trace 
of metamerism. Obviously, the two genera represent very dif- 
ferent modes of origin of dermal spines. That either of them 
occupied a cephalic position, as suggested by Dr. Dean for 
Edestus, seems to the present writer improbable on account of 
their large size, a likelier position being somewhere along the 
middle of the back. Newberry’s idea as to Edestus was that 
the spines were situated “in the position of the second dorsal 
fin on the back or tail of a Plagiostome fish.” 
The light-colored area at the posterior or pointed end of 
Belemnacanthus (Fig. 50, A, C) has been injured somewhat by 
atmospheric erosion, but still permits the course of the vascular 
canals to be seen. These run essentially parallel to the orna- 
mental markings on the lateral faces, and prove that growth 
took place by additions to the posterior end. The markings 
referred to are in the nature of superficial pittings and furrows, 
the latter being sometimes continuous and bifurcating, or again 
short and interrupted. The dotted outline along the crest 
indicates the position of a thin piece of bone that, having 
been accidentally broken off from here, was used for sectioning. 
Nothing very conclusive was gained by this operation, however. 
The projecting portion of matrix at the top of Fig. 50, A, G 
preserves an impression of the base of the spine as far as it 
extends. The lower margin being evenly arched throughout 
its extent of 37 cm., and the summit also as far as it is preserved, 
we note in this another point of resemblance to Edestus. In 
conclusion, it may be worth while to record that the largest 
Arthrodires yet obtained from the Eifel Devonian are Aspi- 
dichthys ingens v. Koenen; Anomalichthys scaber v. Koenen; and 
Dinichthys eifeliensis Kayser; and the largest Elasmobranch the 
