Prof. E. D. Cope on the Systematic Belations of Fishes. 163 



with a number of others, is that which formed the basis of 

 Bonaparte and Mliller's order of Physostomi. The presence of 

 the ductus pneumaticus, which characterizes it, is always asso- 

 ciated with the abdominal position of the ventral fins and with 

 cycloid scales, and mostly with the presence of the precoracoid 

 arch, the entrance of the maxillary bone into the border of 

 the mouth, and the non-separation of the parietal bones by the 

 supraoccipital. Yet none of these characters are precisely 

 associated at the point of change in each ; for there are phy- 

 sostomous fishes with separated parietals and ctenoid scales 

 (some Cyprinodontidte), and there are Physoclysti with abdo- 

 minal ventrals. Nevertheless three prominent types stand 

 out in the Actinopteri — the Sturgeons or Chondrostei, the 

 Physostomi, and the Pliysoclysti, which may be considered 

 tribes. 



An entire series of basilar segments of the abdominal ven- 

 tral fins ; no branchiostegal rays. Chondrostei. 



Basilar segments of ventrals rudimental, position of fins 

 abdominal, parietal bones usually united; branchiostegal rays; 

 swimming-bladder connected with the stomach or oesophagus 

 by a ductus pneumaticus. Physostomi. 



No ductus pneumaticus ; parietal bones separated by the 

 supraoccipital ; ventral fins usually thoracic or jugular ; no 

 basilar segments. Physoclysti. 



Chondrostei. 



There are two orders in this division, as follows : — 



A precoracoid arch ; no symplectic bone ; premaxillary 

 forming mouth -border ; no suboperculum, nor prjeoperculum ; 

 mesopterygium distinct ; basihyals and superior ceratohyal 

 not ossified ; interclavicles present ; no interoperculum or 

 maxillary ; branchio-hyals cartilaginous. Selachostomi (the 

 Paddle-fish). 



Similar to the last, but with interopercle, maxillary bones, 

 and osseous branchio-hyal. Qlaniostomi (the Sturgeons). 



The first order embraces the single family of Spatularichv^ 

 the second that of Acipenseridtc. In both the chorda dorsalis 

 persists, 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 tlie 

 orders of this division : — 



