HORTON — GEOLOGY OV THE STONESFIELD SLATE 
255 
Greek name. These parts arc so characteristic, and preserved in 
such a dof^-ree of perfection, as in most cases to admit of correct 
reference to the faniiUes and genera. The species are all extinct, and 
belong to the Placoid and Ganoid divisions of the four great orders 
of fishes established by Agassiz. Of these there are but few living 
examples, as the fishes that iidiabit the seas of the existing period 
belong, with a limited number of exceptions, to the other two orders, 
Ctenoid and Cycloid, which were not introduced until the Cretaceous 
epoch, an era when the more ancient Placoids and Ganoids had 
already begun to decline. The Placoid fishes of the Stonesfield 
Slate are I'eferable to tlu-ee families, namely, the Cestraciontidaj, 
HybodontidiB, and Edaphontidoa. The Cestraciontida3 are repre- 
sented by species of the genera Acrodus, Aster acantlnis, Stroj^hodus, 
Ceratodus, LeptacantJius, NemacanfJms, and Pristacantlms. The 
teeth of this family form their chief characteristic, and under the 
name of Palates have attracted miich attention from paliiaontologists. 
These palatal teeth are flat and oblong, or quadrangular in shape, and 
often beautifully enamelled. Beneath the enamel, the surface of 
which is frequently worn away, the body of each tooth is composed 
of a strong mass of bone. In some species not less than sixty of 
these teeth were embedded in each jaw, forming a kind of tesselatcd 
pavement, which constituted a most efficient apparatus for crushing 
the shells of Crustacea and moUusca, probably the piincipal food of 
these fishes. The Gestracion FhilUpi, a shai'k that inhabits the Aiis- 
tralian seas, presents the only known analogy to the extinct Acrodi 
and Strophodi of Stonesfield. The palatal teeth most frequently to 
be met with in the slate are those of Acrodus leiodus, and sometimes 
measure one and a half inches in length by three-quarters in breadth. 
These, from their resemblance in fonn and colour to contracted 
leeches, they are regarded as such by the workmen, who are seldom 
without an analogy for any fossil they may offer for sale to the 
stranger geologists who visit their pits. Indeed, so far is the wis- 
dom of philosophy transcended by that of these unlettered sages, 
that in cases where the mere scientific observer can only perceive the 
most ordinary fragments of stone or slate, these village savans can 
frequently succeed in demonstrating most clearly to their oivn satis- 
faction the presence of strange creatures that, like man, must have 
been most fearfully and wonderfully made, if they had ever been 
possessed of existence. Unfortunately, however, for the theories of 
these gifted men, all such specimens are invariably declined with 
thanks by those visitors possessed of the slightest knowledge of 
palfeontology. Many amusing instances of these attempts at restora- 
tion might be related ; we merely allude to them in order to convince 
those who are comparatively young geologists, that a certain degree 
of caution is requisite in their pm'chases of specimens from the 
quarrymen. 
The Hybodontidfe are represented by various sjDecies of Hybodus, 
chiefly recognized by their striated teeth, which are sharp-edged and 
weU adapted for cutting. Many varieties of these are found at 
