CLASSIFICATION. 31 



SUBPHYLUM D. VERTEBRATA. 

 DIVISION (I). CYCLOSTOMATA. 



Order 1. MARSIPOBRANCHII. 

 Family Myxinoidei. Myxine hag-fish. 



Bdellostoma. 

 Family Petromyzontidae. Petromyzon lamprey. 



(Ammocoetes larval lamprey.) 

 Family Palaeospondylidae. Palaeospondylus. 



Order 2. OSTRACODERMI. 

 Suborder 1. HETEROSTRACI. 

 Family Pteraspidae. Pteraspis. 



Suborder 2. OSTEOSTRACI. 

 Family Cephalaspidae. Cephalaspis. 



Suborder 3. ANTIARCHA. 

 Family Asterolepidae. Pterichttiys. 

 Asterolepis. 



DIVISION (II). GNATHOSTOMATA. 

 A. ICHTHYOPSIDA. 



CLASS I. PISCES. 

 Order 1. ELASMOBRANCHII. 

 Suborder (1). ICHTHYOTOMI. 

 Family Pleuracanthidae. Xenacanthus. 

 Suborder (2). PLEUROPTERYGII. 



Cladoselache. 

 Suborder (3). SELACHII. 



Group SQUALIDAE. 



Family Notidanidae. Heptanchus. 

 Hexanchus. 



Chlamydoselache frill-gilled shark. 

 Family Cochliodontidae. Cochliodus. 



