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amphibians, which either lies dorsal to the united trabeculae (ganoids 

 and teleosts) or dorsal to the plane of the trabeculae (amphibians). 



The cornua trabecularum of the ganoid, teleost and amphibian 

 first grow laterally, according to current descriptions, beneath the 

 nasal sacs to form the solum nasi, and later grow upward between 

 those sacs to form the mesial wall or walls of the nasal capsules. In 

 selachians, on the contrary, according to both Parker's (1876) and 

 Sewertzoff's (1899) descriptions of the Scylliidae and Spinacidae, 

 these cornua certainly first grow upward, instead of laterally, and so 

 first form what are at the same time the mesial walls of the nasal 

 capsules and the lateral walls of the cavum praecerebrale, and no 

 subnasal outgrowths are described, or even indicated as such by 

 either of these authors. The detached second labials of Parker's des- 

 criptions have however approximately the positions of these subnasal 

 outgrowths, and if there be not here conditions of classificatory 

 value, it seems probable that in the ScyUiidae and Spinacidae the 

 development of these subnasal cornua must be in some way masked 

 or abbreviated. In embrj^os of Ceratodus (Sewertzoff, 1902), it is 

 to be noted, these cornua are relatively unimportant and lie anterior 

 to instead of ventral to the nasal sacs, the Dipnoi as well as the 

 Elasmobranchii thus apparently differing somewhat in this respect 

 from the Teleostomi and Amphibia. 



But, be this as it may, in the adult Chlamydoselachus the sub- 

 nasal outgrowths of the cornua trabecularum are, to all appearance, 

 definitely found, this outgrowth, on either side, corresponding in 

 position to the solum nasi of Gaupp's descriptions of the Urodela. 

 Eelated to this outgrowth, which is prenasal in position, there is a 

 postnasal one which corresponds in position to the processus antorbi- 

 talis of amphibians, the two outgrowths having the appearance, in 

 the adult Chlamydoselachus, of being parts of a single, subnasal plate 

 that has its origin in the line of the lateral edge of that part of the 

 united trabeculae that lies between the foramina olfactoria, and, in 

 most specimens, is definitely cut into anterior and posterior portions 

 by a deep incisure that lies directly opposite the foramen olfactorium 

 and extends through the entire width of the plate. This incisure, 

 which thus has the position of the fenestra choanalis of amphibians, 

 is closed, in the natural state, by a tough membrane, and it tends to 

 become a closed fenestra by the encroaching ingrowth of the bounding 



