250 FISHES 



stone, and have been compared to the teeth of the Cyclostomes ; 

 but these fossils in microscopic structure do not resemble such 

 teeth, and nothing can be said about them. Small fossils about 

 two inches long found by Dr. Traquair in the Caithness flag- 

 stones near Thurso appear to have the structure of Cylostomes, 

 but they have a calcified internal skeleton, while the skeleton of 

 existing Cyclostomes is soft and cartilaginous. 



The ancient fossils called Ostracoderms are believed to re- 

 semble the lampreys in the absence of jaws and paired fins. 

 These are certainly the oldest known remains of vertebrates, 

 but unfortunately it is impossible to connect them with any of 

 the ordinary types of fishes. Their structure is quite peculiar, 

 and no transitional forms are known leading to sharks or other 

 fishes. The name means shell-skinned animals, and refers to 

 the calcified plates which cover the body, or at least the head. 

 The head region was covered with a large single dorsal plate, 

 in Pteraspis there was a ventral plate also. The under region 

 of the body was covered with series of bony scales somewhat 

 like those of a Ganoid. In Pterichthys there is a pair of jointed 

 limbs at the sides of the head, but these do not correspond to 

 pectoral fins, they are merely the lateral angles of the head- 

 shield produced and jointed. Unpaired fins are, however, pre- 

 sent, but they are formed entirely by parts of the dermal scales. 

 There is a dorsal fin, an asymmetrical (heterocercal) tail fin, 

 and sometimes a ventral fin. These in structure correspond 

 to the dermal part of the fins of fishes : but there is no trace of 

 internal skeleton. The character of the armour, and the groov- 

 ing of the armour-plates for sensory canals indicate that the 

 animals were fishes, and Smith Woodward finds indications of 

 gill-pouches. The Ostracoderms have been found only in the 

 Upper Silurian and the Devonian rocks. In 1859 a specimen 

 of Pteraspis was found in the Lower Ludlow beds in Shropshire, 

 belonging to the Silurian. This fossil was associated with 

 marine shells, especially Cephalopoda or cuttle-fishes, so that 

 it might be concluded that the fish was marine. Pteraspis also 

 occurs in the bone-bed of the Upper Ludlow formation near 

 Ludlow, in which the greater part of the fish remains belong to 

 the Elasmobranchs. This also indicates a marine habitat. Of 

 the English Devonian strata, the beds which occur in Devon are 

 of marine character, while those of Scotland, to which the name 



