DESCRIPTION OF THE FOSSIL REMAINS. 59 



are attached. Hence scales vary remarkably in their form, structure, mode 

 of adhesion, and position in different families. In general, they may be 

 described as flat plates, variously marked and laminated. The plates in 

 some classes are more numerous than in others, the lowest are the largest, 

 and the upper surface becomes in consequence somewhat imbricated. The 

 breadth of each new layer is greater than the last, its edges project farther, 

 and gradually the whole surface assumes that concentric striated appearance 

 which renders it so characteristic a feature in all the scales of the Dura Den 

 fossils. The scales in the genus holoptychius are imbricated, with the distal 

 edge free, and the epidermis enveloping their base. But in other fishes, as 

 glyptopomus and diplopterus, where the scales are remarkably thick, they are 

 arranged laterally, without resting upon each other, and have a covering of 

 the epidermis spread over them, or uniting them at their base. 



The materials of which scales are composed are deposited sometimes in 

 masses, as in the scales of the crocodile; in filaments, as in hairs; and in 

 layers, as in hoofs, nails, and scales of fishes. The annual additions are made 

 to them at their base, or root, where the vessels deposit fresh materials, and 

 they gradually increase in size, protrude and extend along the skin, and 

 continue to grow by the same process as long as these vessels continue in 

 activity. Hence, as by the horns of mammalia, the plates of insects, and the 

 concentric rings in the woody tissue of trees, the age of fishes may likewise 

 be determined by the successive increments of growth to their scaly envelope. 

 The scales of the holoptychius sometimes attain to an enormous size, those 

 from Clashbennie and Parkhill, in the author's possession, exceeding three 

 inches in length, by two inches in breadth, and the wrinkled striag raised in 

 proportional depth. The constituents of a scale from Clashbennie of this 

 genus, analysed by Professor A. Connell, and published in the Edinburgh 

 New Philosophical Journal^ are as follows, viz. : — 



Phosphate of lime, with a little fluoride of calcium, . 91 '42 



Carbonate of lime, ........ 7 '05 



Chloride of potassium, 0-27 



Chloride of water, 0-97 



Sandstone matrix, ........ 2 38 



Phosphate of magnesia and animal matter, trace, 



10209 



The Head. — This organ in the genus holoptychius is proportional and in 

 keeping with the general size of the fish. All the bones are incrusted with 

 a thick coating of enamel, slightly marked, and almost the same as the 

 cephalic bones or the scales of the sturgeons. The external surface is rough, 

 forming a coarse granulation, approaching the deep wrinkles with which the 

 scales are ornamented. The under jaws are large, curved, forming a half 

 circle roimd the head, and incrusted all over with a granulated enamel. The 

 branchiotic rays are supplied, as in most of the ancient sauroids, and as in the 

 living polypterus of the African rivers, by two large enamelled plates uniting 

 at the median line, and filling all the opening of the throat between the two 

 branches of the lower jaw. They are termed the glosso-hyal plates, which 

 are the supporters of the lower jaw, and resemble very much the corresponding 



