HOLOCEPHALI. 155 



temp, and trop. seas, attain great size, 20 ft. wide. Last two genera 

 often called sea-devils. Extinct genera : Ptychodus Ag., Cretaceous ; 

 Promyliobatis Jaekel, Eocene. 



The following extinct Palaeozoic families are placed here : 

 COCHLIODONTIDAE, with several genera, from the Carboniferous Lime- 

 stone ; PSAMMODONTIDAE, from the Carboniferous Limestone ; and 

 PETALODONTIDAE, also from the Carboniferous. 



Order 5 HOLOCEPHALI.* 



Without spiracle, with four clefts covered by an opercular fold 

 which contains a cartilaginous plate. The skull is autostylic, and 

 the notochordal sheath is unsegmented. There are two dorsal fins 

 and an anal. 



The Holocephali differ from the Plagiostomi in the fact that 

 there are only four gill clefts (though there are five branchial 



FIG. 8Q.Chimaera momtrosa (Rdgne Animal). 



arches). Moreover the gill apertures are covered by an opercular 

 fold, and the palatoquadrate bar is continuous with the skull 

 in its whole extent. They have a cartilaginous skeleton and 

 claspers on the pelvic fins of the male. 



The mouth is small, ventral, and bounded by lip -like folds 

 supported by labial cartilages. The nostrils are confluent with 

 the mouth. The urogenital part of the cloaca is separate from 

 the rectum and opens behind the anus. The anterior dorsal fin 

 has a strong spine, on its front border, which is attached to the 

 fused neural spines of the anterior vertebrae. The tail is hetero- 

 oercal and prolonged in Chimaera into a long filament. There 



* G. Good and T. H. Bean, Oceanic Ichthyology, Memoirs of the Museum 

 of Comp. Anat. Harvard College, 22, 1896. A. A. W. Hubrecht, Kent- 

 niss des Kopfskelet d. Holocephalen, Niederldnd Arch. ZooL, 3, 1877. 

 S. Garman, The Chimeroids, Bull. Mus. Harvard Coll., 43, 1904. 



