CYCLOSTOMATA. 55 



anterior part of the body is covered dorsally by a single large 

 shield which differs from those of the Heterostraci in having 

 the inner layer ossified. The middle layer contains canals for 

 the passage of blood vessels, but the exoskeleton shows no 

 impressions of dermal sense organs. The posterior part of the 

 body is covered by large quadrangular scales. Paired append- 

 ages are absent, but median dorsal and caudal fins occur 

 supported by scales, not fin-rays. Cephalaspis, the best known 

 of these animals, occurs in beds of Lower Devonian age. 



Suborder (3). ANTIARCHA. 



The exoskeleton is formed of bony plates, the dorsal and 

 ventral shields each consisting of several symmetrically arranged 

 pieces. The tail may be covered with small scales or may be 

 naked. The head is articulated with the trunk, and its angles 

 are drawn out into a pair of segmented paddle-like append- 

 ages, covered with dermal plates. The orbits are close together. 

 A dorsal fin and traces of mouth parts occur in Pterichthys, 

 but the endoskeleton is unknown. The best known forms 

 Pterichthys 1 and Aster olepis occur in beds of Lower Devonian 

 age. 



GENERAL ACCOUNT OF THE SKELETON OF 



MARSIPOBRANCHII. 



The Marsipobranchii are worm-like animals. The living 

 forms include two families, the Myxinoidei (Hags) genera 

 Myxine and Bdellostoma and thePetromyzontidae (Lampreys). 

 Three species of Petromyzon are known, P. fluviatilis, P. 

 marinus and P. planeri. The larval forms were for a long 

 time thought to belong to a separate genus and were called 

 Ammocoetes. 



The Myxinoids, although very highly specialised in their 

 own way, are at distinctly a lower stage of development than 

 the adult Lamprey, and come nearer to the larval Lamprey or 

 Ammocoete. 



1 See B. H. Traquair, Ann. Nat. Hist., ser. 6, vol. n. 1888, p. 485. 



