INTEODUCTION. ix 



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 maybe 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 (Acipenseridsae) and Paddle-fishes 

 (Polyodontidaj) ; neither family is represented in Mexico or Central America. 



Tn 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 geneva J^ejndosteus 

 and A7nia 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 (Elopidse, Albulidaj, Chirocentridse), and a study of the fossils 

 makes it still more difficult to recognize two orders, annectent forms (e. g. Baijedius, 

 Fholido])ltorus) 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 opisthocoelous 

 vertebrae. 



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 metapterygiupi 

 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.-AMEE., Pisccs, Fclniary 1908, h 



