14 Mr. C. T. Eegan on the 



forming- a communication between tlie air-bladder and the 

 internal ear *. 



Each ductus endolymphaticiis unites with its fellow and 

 gives rise to a backwardly directed median diverticulum, the 

 sinus endolymphaticus, lying in a cavity of the basioccipital 

 which is roofed by laminar inward expansions of the ex- 

 occipitals. This cavity, the cavum sinus imparls^ communi- 

 cates anteriorly with the perilymph spaces surrounding the 

 internal ear, and posteriorly, where its bony roof terminates, 

 gives rise to a pair of lateral cavities, the atria sinus hnparis. 

 The wall of each atrium is partly membranous and is in part 

 formed by two ossicles, claustrum and scaphium, which repre- 

 sent the supra-neural and neural arches of the first vertebra; 

 a ligament connects the scaphium with the tripus, a triangular 

 or triradiate bone mainly formed of the rib of the third 

 vertebra, but in the Characiformes clearly including the para- 

 pophysis also ; in the connecting ligament may appear the 

 zntercalariwn, primarily representing the neural arch of the 

 second vertebra. The air-bladder is typically divided into 

 anterior and posterior portions ; the former is supported by 

 the OS suspensorium, either the parapophysis or rib of the 

 fourth vertebra, or perhaps both, and in its wall is inserted 

 the posterior process of the tripus. The other portions of 

 the tripus and the intercalariwn lie in the cavity of the saccus 

 par aver tebralis, a membranous sac filled with a seraigelatinous 

 fluid ; in the more generalized types this sac communicates 

 anteriorly with the subdural lymph spaces of the cranium 

 through a lateral occipital foramen at the side of i\\Q foramen 

 magnum. 



Sagemehl thought that the Ostariopliysi were related to 

 Amia, but there can be little doubt that they are allied to 

 generalized Clupeoids such as the Elopida3. The Characi- 

 formes are the least specialized of the Ostariophysi, and the 

 absence of a splenial, the presence of an endochondral supra- 

 occipital, the ossified mesocoracoid, the complete vertebral 

 centra, without caudal intercentra, and the homocercal struc- 

 ture of the caudal fin show that they are far removed from 

 Amia and are closely related to the Isospondyli. 



The Ostariophysi may be divided into two well-marked 

 suborders, Oyprinoidea and Siluroidea ; the classification of 

 the former is the subject of the present memoir. The 



* On the Weberian mechanism of the Oyprinoidea see especially 

 Sag-emeh], Morph. Jahrb. x. 1885, p. 102; Sorensen, Vid. Selsk. Skr. 

 Copenhag-en, (6) vi. 1892, p. 131 ; Bridge and Haddon, Phil. Trans. 

 eL\xxiv. (B), 1893, p. 65 ; Bloch, Jenaisch. Zeitschr. xxxiv. 1900, p. 1. 



