9^3 



CHAPTER XLVII. 



CLASS PIS C E S — continued. 



Orders Cyclostomi and Elasmobranchei. 



Order I. Cyclostomi. — The Cyclostomi, which include the 

 Hag-fishes (Myxine and Bdellostomd) and Lampreys (Petromyzon) 

 being at present unknown in a fossil state, require no further notice 

 in the present work. 



Order II. Elasmobranchei. — This order, which is also known 

 under the name of Chondropterygii, includes a peculiar extinct 

 group termed the Ichthyotomi, together with the modern Sharks, 

 Dog-fishes, Saw-fishes, and Rays, collectively constituting the Se- 

 lachii, and all of which are typically of marine habits. For palaeon- 

 tological purposes this order may be characterised as follows : The 

 skeleton is invariably cartilaginous, and membrane bones are, with 

 some possible exceptions, absent in the skull ; the vertebral column 

 is, however, generally divided into distinct segments, of which the 

 centra may be marked by a calcification differing in structure from 

 true bone. In the skull, which may be either movably or immov- 

 ably connected with the vertebral column, the palatopterygoid bar 

 and hyomandibular suspensorium are never fused with the cranium. 

 When an exoskeleton is developed, it consists of small dermal gran- 

 ules, of which the structure is the same as that of teeth. In all 

 existing forms the optic nerves simply cross one another, without 

 any interlacing of their component fibres ; the bulbus arteriosus of 

 the heart has three series of valves ; the intestine is furnished with 

 a spiral valve ; and the ova are of large size and few in number. 



A few words may be said explanatory of some of the above-men- 

 tioned and other features in existing forms before proceeding to the 

 systematic part. In all forms, as already mentioned, there is a 

 separate suspensorial arrangement articulating with the cranium, to 

 which the mandible is attached ; this structure being termed hyosty- 

 lic. Usually there is a hyomandibular suspensorium intervening 



