4i8 



MANUAL OF ZOOLOGY. 



b. Pteraspis. In most respects this genus was not unlike 

 Pterichthys, but it did not possess the peculiar pectoral fins of 

 the latter. More than one species of Pteraspis have been 

 found in the Upper Silurian Rocks (Ludlow), and are as yet 

 among the earliest known indications of the appearance of the 

 great sub-kingdom Vertebrate upon the globe. 



c. Cephalaspis (fig. 176). This, again, is not unlike Pterich- 

 thys in many respects. The cephalic buckler, however, has 

 its posterior angles produced backwards, so as to give it the 

 shape of a " saddler's knife/' whilst the pectoral limbs have not 

 the form of spines. 



d. Coccosteus (fig. 177). This is another characteristic 

 genus of the Old Red Sandstone. In this genus, as in the 

 preceding, there is a cephalic buckler, the plates of which are 



Fig. 177. i. Coccosteus decipiens ; 2. Pterichthys Miller i. 



covered with small hemispherical tubercles. The notochord 

 was persistent, but the neural and haemal spines, and the rays 

 of the dorsal and ventral fins, are well ossified. A large hetero- 

 cercal tail-fin was doubtless present as well. 



CHAPTER LVIII. 



ELASMOBRANCHII AND DIPNOI. 



ORDER V. ELASMOBRANCHII ( = Selachia, Miiller ; Placoidei, 

 Agassiz ; Holocephali and Plagiostomi* Owen). This order in- 

 cludes the Sharks, Rays, and Chimaerse, and corresponds with 

 the greater and most typical portion of the Chondropterygida or 

 Cartilaginous fishes of Cuvier. The order is distinguished by 

 the following characters : The skull and lower jaw are well 

 developed, but there are no cranial bones, and the skull con- 



