54 THE VERTEBRATE SKELETON. 



persistent notochord enveloped in a sheath. The notochord 

 in living forms is unsegmented, but segmented cartilaginous 

 neural arches are present in some cases. A complicated series 

 of cartilaginous elements occurs in relation to the mouth, gills, 

 and sense organs. The median fins are supported by carti- 

 laginous pieces, the radialia. The order includes the Lampreys 

 and Hags. 



Order II. OsTRACODERMi 1 . 



The forms included in this group have long been extinct, 

 being known only from beds of Upper Silurian and Lower 

 Devonian age. They differ much from all other known animals. 

 The exoskeleton is always greatly developed and includes 

 (1) large bony plates covering the anterior region; (2) scales 

 covering the posterior region. The plates are deeply marked 

 by canals belonging to dermal sense organs. Jaws are un- 

 known, and arches for the support of the appendicular skeleton 

 are rudimentary or absent. The tail is heterocercal (see p. 60). 



Suborder (I). HETEROSTRACI. 



The exoskeleton consists principally of calcifications form- 

 ing dorsal and ventral shields which cover the head and ab- 

 dominal region; the dorsal shield is formed of a few plates 

 firmly united, the ventral shield of a single plate. The shields 

 are composed of three layers, the middle layer being traversed 

 by canals belonging to the dermal sense organs which open to 

 the exterior by a series of pores. The tail is sometimes covered 

 by scales. The orbits are widely separated and laterally 

 placed. Paired appendages are absent. These curious forms 

 are found in beds of Upper Silurian and Lower Devonian age. 

 One of the best known genera is Pteraspis. 



Suborder (2). OSTEOSTRACI. 



The exoskeleton as in the Heterostraci consists of shields 

 and scales, the shields being divisible into three layers. The 



1 See A. Smith Woodward, Catalogue of Fossil Fish' in the British 

 Museum, Part n. f 1891. A. Smith Woodward, Nat. Sci. vol. i. 1892, 

 p. 596. 



