68 THE VERTEBRATE SKELETON. 



developed. Intercalary pieces (interdorsalia) occur between 

 the neural arches, and similar pieces (interventralia) between 

 the haemal arches. The cranium is covered with membrane 

 bone, and teeth are but slightly developed. The tail is hetero- 

 cercal. Gill rays occur on the hyoid arch, and the gills are 

 protected by a bony operculum attached to the hyomandi- 

 bular. The skin (1) may be almost or quite naked, (2) may 

 carry bony plates arranged in rows, or may be covered (3) with 

 rhomboidal scales, or (4) partly with rhomboidal, partly with 

 cycloidal scales. 



Suborder (2). CROSSOPTERYGII. 



The exoskeleton has the form of cycloidal or rhomboidal 

 scales. The condition of the vertebral column differs in the 

 different genera. Sometimes, as in Polypterus, there are well- 

 developed ossified vertebrae; sometimes, as in many extinct 

 forms, the notochord persists and is unconstricted. The tail 

 may be diphycercal *or heterocercal. The pectoral and some- 

 times the pelvic fins consist of an endoskeletal axis bearing 

 a fringe of dermal rays. 



Suborder (3). HOLOSTEI. 



The exoskeleton has the form of cycloidal or rhomboidal 

 scales. The notochord is constricted and its sheath is seg- 

 mented and ossified, forming distinct vertebrae, which are 

 generally biconcave, sometimes opisthocoelous (Lepidosteus) . 

 The cartilaginous cranium is largely replaced by bone, and in 

 connection with it we find not only membrane bone, but 

 cartilage bone, as the basi-occipital, exoccipitals, and pro- otic 

 are ossified. The tail is heterocercal. The suspensorium 

 resembles that of Teleosteans, consisting of a proximal ossifi- 

 cation, the hyomandibular, which is movably articulated to 

 the skull and a distal ossification, the symplectic. The two 

 are separated by some unossified cartilage. The cartilaginous 

 upper and lower jaws are to a great extent surrounded and 

 replaced by a series of membrane bones. 



