STRUCTURES OF CHIM&ROIDS 



103 



morphosed ; the mandible appears to be autostylic, or artic- 

 ulated directly with the skull cartilage, PQ. The gill 

 arches are shark -like, but the hyoid arch appears far less 

 modified than in sharks ; its upper element, HM, is thus 

 unconnected with either the skull or the joint of the jaw ; 

 its distal element, CH, has, however, developed a series of 

 specialized supports for the dermal gill shield, OP. The 

 study of the fin supports shows the dorsal elements, B + R, 

 representing probably the radial and basal elements to- 

 gether, arranged in a single row margined distally by the 

 longitudinal ligament, LL, supporting the dermal func- 

 tional fin, D. The paired fins are readily reduced to the 

 plan of those of Fig. 84 ; their girdles, however, seem to 

 have acquired more modified characters, their ventral and 

 dorsal elements greatly increasing in size. 



Chimaeroids as a group have received but a small share 

 of the attention paid to the other fishes ; their living 

 forms are few and comparatively rare ; their embryology 

 and larval history are unknown ; and their life habits have 

 been suggested only in the work of Dr. Giinther (Chal- 

 lenger Report}. His record of the taking of immature 

 specimens of Chim&ra at great depths seems thus far the 

 most important clue as to the conditions of their living 

 and breeding.* 



Fossil Ckimceroids 



Fossil Chimaeroids have left behind them very imperfect 

 records of the history of their group. Like the sharks, 

 little more than their dental plates and fin spines have 

 usually been preserved. The structures of some of their 

 ancient members appear to have differed little from those 

 just described in the recent Chimaera. In Ischyodus, 



* Cf. also Goode and Bean, on Harriotta, P. U. S. Nat. Mus., XVII. 471-473. 



