INTRODUCTION. 1x 
the branchiostegal rays replaced by a pair of large gular plates (absent in some 
specialized Dipneusti), the paired fins more or less lobate, their basal supports 
becoming axial in the more specialized forms, and the duct of the air-bladder opening 
into the ventral part of the cesophagus. The Actinopterygian series includes two 
main groups which may be given ordinal rank; in the lower (Chondrostei) the clavicles 
(infra-clavicles) are distinct from the cleithra (clavicles), the pelvic fins have a well- 
developed series of radials, the median fins have the dermal rays more numerous than 
their endoskeletal supports, and the caudal fin is typically strongly heterocercal. The 
living members of this order are the Sturgeons (Acipenseride) and Paddle-fishes 
(Polyodontidee) ; neither family is represented in Mexico or Central America. 
In the more specialized group, the Teleostei, the clavicles do not exist as separate 
elements, the radials of the pelvic fins are absent or vestigial, the dermal rays of the 
median fins are equal in number to their endoskeletal supports, and the caudal fin is 
abbreviate heterocercal or homocercal. 
The Teleostei are the dominant group to which the great majority of living fishes 
belong ; their classification is by no means an easy matter. The genera Lepidosteus 
and Ama differ from other living Teleosts in the presence of a splenial and of 
a metapterygium, and in the absence of an endochondral supraoccipital ossification ; 
they have been regarded as belonging to a separate order, Holostei. 
Some of the supposed distinctive features of the Holostei have been found in 
undoubted Teleosts (Elopide, Albulide, Chirocentride), and a study of the fossils 
makes it still more difficult to recognize two orders, annectent forms (e. g. Dapedius, 
Pholidophorus) occurring. 
Lepidosteus is represented in the fresh waters of Mexico and Central America ; 
it is the type of the sub-order Ginglymodi, characterized by the opisthoccelous 
vertebra. 
The remaining Teleostei have a well-ossified endochondral supraoccipital, the 
lower jaw composed of three elements only (dentary, articulare, and angulare), and the 
pectoral radials all directly attached to the scapula and coracoid, the metapterygium 
being absent. 
The most generalized of these form the sub-order Malacopterygii, soft-rayed fishes 
with abdominal ventral fins, with a pneumatic duct, and with a mesocoracoid element 
in the pectoral arch. ‘This sub-order is represented in the fresh waters of Mexico and 
Central America by a few marine types. 
BIOL. CENTR.-AMER., Pisces, February 1908. b 
