132 



HISTORICAL PALAEONTOLOGY. 



The last and highest group of the Mollusca that of the 

 Cephalopoda is still represented only by Tetr abranchiate 

 forms; but the abundance and variety of these is almost 

 beyond belief. Many hundreds of different species are known, 

 chiefly belonging to the straight Orthoceratites, but the slightly- 

 curved Cyrtoceras is only little less common. There are also 

 numerous forms of the genera Phragmoceras, Ascoceras, Gyro- 

 ceras, Lituites, and Nautilus. Here, also, are the first-known 

 species of the genus Goniatites a group which attains con- 

 siderable importance in later deposits, and which is to be 

 regarded as the precursor of the Ammonites of the Secondary 

 period. 



Finally, we find ourselves for the first time called upon to 

 consider the remains of undoubted vertebrate animals, in the 

 form of Fishes. The oldest of these remains, so far as yet 

 known, are found in the Lower Ludlow rocks, and they con- 

 sist of the bony head-shields or bucklers 

 of certain singular armored fishes belong- 

 ing to the group of the Ganoids, repre- 

 sented at the present day by the Stur- 

 geons, the Gar-pikes of North America, 

 and a few other less familar forms. The 

 principal Upper Silurian genus of these is 

 Pterasf>is,and the annexed illustration (fig. 

 74) will give some idea of the extraordi- 

 nary form of the shield covering the head 

 in these ancient fishes. The remarkable 



Fig. 74.-Head-shield of stratum near the top of the Ludlow for- 

 Pterospis Banks li, Ludlow 

 rocks. (After Murchison.) mation known as the bone-bed has 



also yielded the remains of shark-like 

 fishes. Some of these, for which the name 



of Onchus has been proposed, are in the form of com- 

 pressed, slightly-curved spines (fig. 75, A), which would appear 



Fig. 75. A, Spine of Onchus tenuistriatua ; B, Shagreen-scales of Thelodus. Both 

 from the " bone-bed " of the Upper Ludlow rocks. (After Murchison. ) 



to be of the nature of the strong defensive spines implanted 

 in front of certain of the fins in many living fishes. Besides 

 these, have been found fragments of prickly skin or shagreen 

 (SpJiagodus}, along with minute cushion-shaped bodies (Thelo- 



