588 SYSTEMATIC RELATIONS OF FISHES. 
bladder connected with the stomach or w@sophagus by a ductus 
pneumaticus. Physostomi. 
No ductus pneumaticus ; parietal bones separated by the supra- 
occipital ; ventral fins usually thoracic or jugular; no basilar seg- 
ments. Physoclysti. 
CHONDROSTEI. 
There are two orders in this division, as follows : 
A præcoracoid arch ; no symplectic bone ; premaxillary forming 
mouth border; no suboperculum nor preoperculum ; mesoptery- 
gium distinct ; basihyals and superior ceratohyal not ossified ; in- 
terclavicles present; no interoperculum nor maxillary; branchi- 
hyals cartilaginous. Selachastomi—The Paddle-fish. 
Similar to the last, but with interopercle, maxillary bones, and 
osseous branchihyal. Glaniostomi—'The Sturgeons. 
The first order embraces the single family of Spatularide, the 
second that of Accipenseride. In both the chorda dorsalis per- 
sists, the tail is heterocercal and the osseous cranium is little 
developed. The basal and radial elements of the limbs, with the 
coracoids, are not ossified. 
PHYSOSTOMI. 
The following key will’ express the leading features of the 
orders of this division : 
I. A precoracoid arch. 
A. A coronoid bone. 
Maxillary in many pieces; vertebra opisthocoelian. 3. Gingly- 
moui— The Bony Gar. ` 
Maxillary not transversely divided ; vertebrae amphicoelian. 4. 
Halecomorphi — The Dog Fish. 
AA. No coronoid bone. 
* No symplectic bone. 
Pterotic simple, anterior vertebrae with ossicula auditus ; supra- 
occipital and parietals coéssified. 5. Nematognathi— The Cat 
ishes. 
Pterotic annular, including a cavity closed by a special bone; 
parietals distinct, vertebrae simple. 6. Scyphophori—The Mor- 
myri, 
** Symplectic present. 
Anterior vertebree codssified and with ossicula auditus. 7. Plec- 
tospondyli— The Suckers, etc. 
eee on ed eae 
