SYSTEMATIC RELATIONS OF FISHES. 587 
Opercular bones, well developed on separate and complex sus- 
pensorium; a double ceratohyal, no pelvic elements. Primary 
radii of fore limb parallel with basilar elements, both entering the 
articulation with scapular arch. Basilar elements reduced to met- 
apterygium and very rarely mesopterygium. Primary radii of 
posterior limbs generally reduced to one rudiment. Actinopteri. 
II. ON THE ACTINOPTERI. 
In determining the primary types of this sub-class, we return 
to some characters already mentioned, in which they approximate 
the Crossopterygia, and adding others, follow the various diver- 
gences to their specialized terminations. 
Thus in Accipenser and allies, the ventral fins possess a complete 
series of basal radial bones, and the pectorals each a large mesop- 
terygium. In Amia and Lepidosteus the mesopterygium is small, 
and the basal radii of the ventrals are reduced to their lowest 
number. In none of them are the basihyals fully developed. 
Most of the eels retain a character which we have only observed 
heretofore in the Selachii. 
We pass by a number of the lower fishes before we find the 
mandibular arch furnished with a symplectic. One of the most 
important modifications, which is more or less coincident with a 
number of others, is that which formed the basis of Bonaparte 
and Miiller’s order of Physotomi. The presence of the ductus 
pneumaticus which characterizes it, is always associated with the 
abdominal position of ventral fins and cycloid scales, and mostly 
with the presence of the przecoracoid arch, the entrance of the 
maxillary bone into the border of the mouth and the nonsepara- 
tion 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 physostomous fishes with separated parietals 
and ctenoid scales (some Cyprinodontide), and there are Physo- 
clysti with abdominal ventrals. Nevertheless, three prominent 
types stand out in the Actinopteri, the sturgeons or Chondrostei, 
the Physostomi, and the Physoclysti, which may be considered-as 
tribes. 
An entire series of basilar segments of the abdominal ua 
fins ; no branchiostegal rays. Chondrostei. 
Basilar segments of ventrals rudimental, position of fins ab- 
dominal, parietal bones usually united ; branchiostegal rays ; swim 
