520 The Class Elasmobranchii or Shark-like Fishes 



Order Ichthyotomi. — In the order Ichthyotomi {ix^vs, fish; 

 TO)xbi, cutting ; named by Cope from the supposed segmentation 



Fig. 307. — Pleuracanthus decheni, restored. (After Brongniart.) The anterior anal 



very hypothetical. 



of the cranium; called by Parker and Haswell Pleuracanthea) 

 the very large pectoral fins are developed each as an archip- 

 terygium. Each fin consists of a long segmented axis fringed 



on one or both sides with fin- 

 rays. The notochord is very 

 simple, scarcely or never con- 

 stricted, the calcifications of its 

 sheath "arrested at the most 

 primitive or rhachitomous stage, 

 except in the tail." This is 

 the best defined of the orders of 



Fig. 308. Fig. 309. 



Flo. 308. — Head-bones and teeth of PleuTacanthus decheni Goldfuss. (After 



Davis, per Dean.) 

 Fig. 309. — Teeth of Didymodus hohemicus Quenstadt. Carboniferous. Family 



Pleuracanlhidtv. (After Zittel.) 



sharks, and should perhaps rank rather as a subclass, as the 

 Holocephali. Two families of Ichthyotomi are recognized by 

 Woodward, the PleuracanthidcB and the Cladodontida . In the 

 Pleuracanthida the dorsal fin is long and low, continuous from 

 head to tail, and the pectoral rays are in two rows. There 

 is a long barbed spine with two rows of serrations at the nape. 



