^i'lSH-EE MAINS OF THE CAEBONIFEEOUS ROCKS. 85 



of these wrinkles the crown was gradually worn smooth, and 

 separate generic and specific names {e.g.^ Helodus subteres) 

 have been given to such worn teeth. The genus Orodiis 

 probably differs but slightly from the Mesozoic genus 

 Ilybodus, the teeth of which are common in the Khsetic of 

 Aust Cliff. Three species — Orodus rainosuSj 0. sculptus^ 

 and 0. cinctiis — have been found in the Bristol limestone. 



In passing to the family Petalodontidcc, we pass to fishes 

 which were most probably depressed like the existing rays, 

 and had large wing-like pectoral fins produced forwards in 

 the direction of the head. The teeth covered the jaws in the 

 form of a pavement of overlapping petal-shaped or ribbon- 

 shaped flattened denticles, the free points or edges of v/hich 

 were directed backwards. The edges were smooth or 

 slightly crenulated, and the root or base of insertion was 

 relatively large. Teeth of Petalodus linearis, narrovv^ and 

 rectangular, with fine crenulations over the free long edge, 

 have been found in the palate bed of the lower limestone 

 shales, and less abundantly in the black-rock limestone. 

 P. recurviis is a rarer form. A specimen in the British 

 Museum (Nat. Hist.), labelled P. Hastingsice (Owen), is re- 

 ferred by Mr. Smith Woodward to P. acuminatus (Ag.). 



Another extinct family, only known by parts of its den- 

 tition, is that of the Psammodontidcc . The teeth are quad- 

 rangular and flattened with a root or base of insertion nearly 

 as large as the crown or hardened crushing substance. They 

 aj^pear to have been arranged in one or more longitudinal 

 rows, probably alternating somewhat in adjacent rows. 

 Teeth belonging to the typical Psammodus are generally 

 oblong, though sometimes nearly square. The base is 

 thicker than the crown, and easily separable from it. Trans- 

 verse wrinkles in some species mark the crown. In this 

 fish the slit-like mouth was probably but slightly arched 



