168 HOLOCEPHALI 



of this family are still obscure ; usually they are associated with the 

 Myliobatidae, but possibly they are more closely related to the Squatinidae 

 (Jaekel). 



Petalodus, Owen ; Potyrkizodus, M'Coy ; Janassa, Miinster ; Carboni- 

 ferous, Europe and North America. 



Order 2. HOLOCEPHALI. 



The Holocephali, a very ancient group of highly specialised 

 marine fish, date from the Devonian, and flourished especially in the 

 Mesozoic times. At the present day they are represented only by 

 a few genera scattered over the world. 



In their general organisation they closely resemble the 

 Selachians : the skeleton is mainly cartilaginous and without true 

 bone ; the two dorsal, the anal, the caudal, and the paired fins are 

 all provided with typical ceratotrichia ; there are pelvic ' claspers ' 

 in the male. Moreover, in living forms the egg is large, heavily 

 laden with yolk, and laid in a horny capsule secreted by the oviduct 

 (Fig. 91, I). The alimentary canal with its spiral valve, the heart 

 with its contractile conus and several rows of valves (Lankester [277]), 

 the urinogenital organs, in fact, all the viscera, are built on the 

 Selachian plan. But the Holocephali have become much specialised 

 in many respects, though in some few points they have perhaps 

 preserved some very primitive characters. 



They differ from the Selachian, in the absence of a rectal 

 gland, which may, however, be represented by glandular patches ; 

 in the absence of an attenuated anterior ' genital ' portion of the 

 female meson ephros ; in the slight differentiation of the stomach, 

 and the small number of turns made by the spiral valve. Lastly, 

 the development is in the main quite Selachian in character ; but 

 the cleavage is of the holoblastic type. 



The skin in the adult of modern genera is smooth over the 

 general surface of the body. A covering of powerful denticles was 

 present in such early forms as Squaloraja and Chimaeropsis (Fig. 

 137, B) ; they are usually preserved in living Holocephali only 

 on the claspers and frontal prehensile process of the male, and may 

 also be present in rows on the head and back of the young as in 

 Callorhynchus. 



They have no enamel, but an outer layer of vitrodentine as 

 in Selachians, and the pulp-cavity is almost filled up. The large 

 spine, almost universally present on the front edge of the first 

 dorsal fin, may be considered as a modified denticle ; it consists of 

 vasodentine. In the Myriacanthidae dermal plates of similar 

 structure are developed on the head ; these have also probably 

 been derived from denticles (A. S. Woodward [506]). 



