FISHES OF THE CARBONIFEROUS SYSTEM 97 
It may seem strange that the muzzles only of Holodus and Ganorhyn- 
chus, disconnected from other portions, should be found in several countries, 
so that we are left in doubt as to the character of the major part of the head 
and all the body in these fishes; but it should be remembered that the 
dental apparatus of most animals is composed of the densest and most dura- 
ble tissues, and it is therefore very frequently preserved, while all other 
parts have perished. In many ancient fishes the dentary element of the 
lower jaw was composed of firm and resistant bone, while the angular and 
articular portions consisted of cartilage, and have entirely disappeared. In 
the economy of nature hardness and strength are given to organs where 
these are necessary and indispensable qualities, while those parts not ex- 
posed to violence or wear consist of soft porous bone or even cartilage. 
This is conspicuously true of the structure of the Elasmobranch fishes, and 
we find the same thing in a less degree throughout the animal kingdom. 
PHYLLOLEPIS DELICATULA, 0. sp. 
Plate XIX, Fig. 11. 
Scales or scutes thin, one inch four lines in length by one inch in 
width, elliptical in outline; the surface marked with fine lines, which on 
the sides are parallel, but in the central portion of either end are somewhat 
confused and reticulate. In the center of the plate the lines are very closely 
approximated, more widely separated on the sides, and still more so at the 
ends. 
The plate upon which this description is based was evidently very 
thin, and the markings on it are delicate. In form and ornamentation it 
exhibits a great similarity to the fossil described by Agassiz’ and called 
Phyllolepis concentricus. At the same time the pecular style of ornamenta- 
tion is almost identical with that of Holonema rugosa, and I can not but think 
that these rounded, detached, thin plates were in some way associated with 
large angular ones, which were united to form a carapace somewhat like 
that of Pterichthys. 
' Poissons Fossiles du Vieux Grés Rouge ou Systéme Dévyonien (Oid Red Sandstone), ete., p. 67, 
pl. 24, fig. 1. 
MON XVI a 
