^RESIDENT'S ADDRESS. Iv. 



teeth vary greatly in structure ; they are either borne on the 

 margin of the jaws or as flattened discs, attached to the roof of 

 the mouth. The borders of the fins are frequently furnished 

 with modified scales known as fulcra. The Order now comprises 

 only seven living genera, Lepidosteus, Polypterus, Calamoichthys, 

 Amia, Accipenser, Scaphirhynchiis, and Spatularia, which are 

 partially or wholly confined to fresh water lakes and rivers, and 

 are confined in the Northern Hemisphere. The terminal 

 part of the notochord is not ossified. The exo-skeleton of 

 Ganoids presents most extreme variations, Spahdaria is naked ; 

 Accipenscr and Scaphirhynchus develope numerous dermal plates, 

 composed of true bones. The single genus Amia, Avhich lives 

 in the fresh-waters of North and Central America, is covered 

 with cycloid scales, which not only overlap, but are fitted 

 .together with pegs and sockets. The endo-skeleton also differs 

 materially. Spatularia, Scaphirhynchiis, and Accipenser have a 

 persistent notochord, the sheath cartilaginous, showing rudi- 

 ments of the vertebral arches. The vertebrae of Polypterus, 

 Lepidosteus, and Amia are fully ossified. 



Palceoniscidce. This family is extinct, and for the most part it 

 is Palaeozoic. It made its first appearance in. the Old Red Sand- 

 stone, and disappeared at the commencement of the Jurassic 

 Age*. 



Cephahisptdw. The members of this group have the head and 

 the anterior part of the body covered with a continuous shield, 

 while the rest is studded with small angular plates or scales. 

 There are no traces of an endo-skeleton, of a lower jaw, or teeth. 

 The buckler is semi-circular in shape, the posterior sides ter- 

 minating in some cases in two long spines pointing backwards, 

 and truncate. There is not enough known at present about 

 these fishes to warrant any conclusions as to their distribution 

 in time or space. Cephalaspids occur in the Lower and Upper 

 Ludlow-beds, in the Downton Sandstone, in the Upper 

 Silurians of Russia and Gallicia, and in the lowest portion 

 of the Old Red Sandstone, assigned by geologists to the 

 Silurian. 



