| OCR Text |
Show 7 2 6 MR. C. TATE REGAN ON [June 19, The relations of the principal groups may be diagrammatically expressed thus:- Narcobatoidei. \ Batoidei. (Hypotremata.) (Selachii Trematopnea.) The diagnostic features of the two series may be compared as follows:- Trematopnea. Gill-clefts opening directly to the exterior. Pterygo-quadrate distinct from the cranium. Chasmatopnea. Gill-clefts opening into a chamber with a single external aperture. Pterygo-quadrate fused with the cranium. or infraliaemals are the ventral counterparts of the supraneurals is shown especially well in the Dipneusti, the ribs meeting in the caudal region to form haemal spines, so that the ventral arches are exactly similar to the dorsal ones. In the Cliondrostei infrahsemal elements are wanting in the posterior abdominal and caudal regions. In fishes with a bony vertebral column the presence of centra and the co-ossification of the neural and supraneural and in the caudal region of the haemal and infrahsemal elements usually obscure the structure. I would, then, define the Selachians as having neural and haemal arches only, and the Teleostomes as having not only neural and haemal arches, but also supraneural and infrahsemal arches. |
