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 Chim&ra, ten inches long in Callorhynchus. 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 ing to the unpublished studies of Dr.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 Chimcera, Callorhynchus, and Harriotta are 

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

 Chimcera rnonstrosa 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, Eumylodus, Rhynchodes, Edaphodon, 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 

 PTYCTODOXTID/E {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 ; no claspers ; skull hyostylic ; 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. 



