242 CLASSIFICATION^ OF VERTEBRATES. 



a common ostium tubae. Of the development almost nothing 

 is known. The eggs are very large, six and one-half inches 

 long in Chinicera, ten inches long in Callorhytichus. Accord- 

 ing to the unpublished studies of Dr. Dean, in the early stages 

 these forms are decidedly shark-like, and the elevation of the 

 division to the rank of a sub-class is not warranted. The 

 pterygoquadrate is free as in other elasmobranchs ; the gills are 

 not covered, and external gill filaments are present. 



Only three genera — Chimara, Callorkytukus, and Harriotta — are 

 known from existing seas. The first is represented on our east coast by 

 Chimara monstrosa and C. affinis, and on the Pacific by C. collei. Harri- 

 otta has been found in the North Atlantic and in Japan. The third genus 



Fig. 244. Chimcera monstrosa, ' king of the herrings.' 



is from the southern seas, and also occurs fossil in New Zealand. Several 

 fossil genera — Ischyodus, Eiimylodus, Rhynchodes, Edap/wdon, etc. — 

 range from the Devonian to the cretaceous. The fossil Squaloraia from 

 the lias of England also appears to belong here. Less certain is the group 

 Ptvctodontid^ (Ptyctodus, Rhynchodus) from the Devonian, known only 

 from the dental plates, a pair in each jaw. 



SUB-CLASS II. TELEOSTOMI. 



Fishes in which bones are developed ; gill slits 5, exter- 

 nally covered with a bony operculum ; scales, when present, 

 ganoid, ctenoid, or cycloid ; n6 claspers ; skull h)ostylic ; upper 

 jaw formed by membrane bones ; skull with sutures ; air-bladder 

 frequently present. 



Here belong all the common fishes, — trout, cod, herring, 

 shad, eels, etc., — as well as a series of forms not so familiar, 

 which are frequently grouped together as ganoids. All these 

 agree in a number of particulars of considerable importance. 



