92 LOWER VERTEBRATES. 



Order I. — ACANTHODINI. 



This order seems to represent the connecting links between the ganoids and the 

 Selachii, or sharks and skates. The forms belonging to it (embracing the genera 

 Acanthodes, Chiraccmthus, Dijilacanthus, etc.) flourished in the Devonian and car- 

 boniferous periods of geological historj'. They had cartilaginous skulls, heterocercal 

 tails, rhomboidal scales which were so small that they gave the exterior a shagreened 

 ajjpearance, and before each fin they were armed with a large spine. 



Order II. — PLACODERMI. 



The members of the Placodermi (or, as the order is sometimes termed, Phractoso- 

 mata) were armored fishes, which had the head and thoracic region enclosed in great 

 bony plates, the external surface of which was variously sculptured. These plates 

 were closely united, so that the whole formed a perfect j)rotection to the viscera. In 

 some the tail was naked, but in others it was covered with ganoid scales. The armor 

 also extended to the pectoral fins, which in some forms were so enclosed in the hardened 

 plates, jointed for motion, that they resembled the appendages of a crustacean. The 

 bodies of the vertebrae were not ossified. These fossils are found in the Silurian and 

 Devonian rocks, and are the oldest vertebrates whose remains are known to the geol- 

 ogist. Some of the forms were very large. Diiiic/Uhi/s, a genus from the Devonian 

 strata of Ohio, reached a length of from fifteen, to eighteen feet, while Astro- 

 lepis, from the corresponding rocks of Eui-ope, measured between twenty and thirty 

 feet. Some doubt exists regarding the position of Dinidtthys ; Professor J. S. Xew- 

 bury, from a consideration of the teeth, regarding it as related to the lung-fishes 

 (Dipnoi). Two families are recognized, the first, Ptekichthid^, having the head 

 covered by several bony plates, the other, the Cephalaspid^, having but a single 

 large plate in the same region. 



Order III. — CHONDROSTEI. 



The sturgeons first appear in rocks of the eocene age, and hence, so far as we 

 know, are the latest group of ganoids to appear. They are distributed through the 

 waters of the northern hemisphere, only four genera and less than twenty-five species 

 being known. In all the skeleton is largely cartilaginous, and the notochord jiersists 

 through life ; the skull is cartilaginous and covered with membrane bones. The skin 

 in some is naked, in others it is covered with bony plates in the place of scales. The 

 tail is heterocercal, and the caudal fin is armed in front with the bony spines or scales 

 already described under the name fulcra. Two families (by some regarded of higher 

 rank, and called Selachostomi and Glanostomi) exist at the jiresent day. 



In the Acipenserid.« we find fishes with elongate bodies ]n-otected with five rows 

 of large bony plates, one running along the dorsal line of the body, one on each njjper 

 lateral and each lower lateral surface. Each plate has a longitudinal ridge or carina 

 crossing its middle, and terminating in a more or less acute spine. Between these 

 large plates are smaller irregular plates, which render the surface i-ough. The snout 

 is flattened and extends to some distance in front of the mouth, and from its lower sur- 

 face hang do^vni four flexible filaments or barbels, arranged in a transverse row. These 



