HOETON — GEOLOGY OF THE STONESFIELD SLATE. 



255 



Greek name. Tliese parts are so cliaracteristic, and preserved in 

 sucli a degree of perfection, as in most cases to admit of correct 

 reference to the families 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 inhabit 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 referable to three families, namely, the Cestraciontidee, 

 Hybddontidee, and Edaphontid^. The Cestraciontidge are repre- 

 sented by species of the genera Acrodus, Aster acantliiis, Stro^liodiis, 

 Ceratodus, Leptacantlius, NemacantJms, and Pristacantkus. The 

 teeth of this family form their chief characteristic, and under the 

 name of Palates have attracted much attention from paleontologists. 

 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 tesselated 

 pavement, which constituted a most ef&cient apparatus for crushing 

 the shells of Crustacea and moUusca, probably the principal food of 

 these fishes. The Gestracion FMllipi, a shark that inhabits the Aus- 

 tralian seas, presents the only kno^vn 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 form 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 own 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 

 palgeontology. 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 purchases of specimens from the 

 quarrymen. 



The Hybodontidge are represented by various species of Hybodus, 

 chiefly recognized by their striated teeth, which are sharp-edged and 

 well adapted for cutting. Many varieties of these are found at 



