PLATOSOMIA. 257 



Evidently this form is very closely allied to R. japonica. Known from the 

 description taken from a Japanese specimen of 580 mm. 



PLATOSOMIA. 



This division includes all of the Chondropterygia in which the body, head, 

 and pectorals are depressed, broadened and joined as so to form a disk on which 

 the eyes and the spiracles are superior and the nostrils, mouth, and gill openings 

 inferior, and from which the tail is more or less distinct and bears upon its upper 

 surface the dorsal fins, when present. The number of fins varies : — in many forms 

 there are two dorsals, in some there is but one dorsal, and in others there are no 

 dorsals; the caudal is present in many genera and absent in many others. An- 

 anal fin does not occur in the division. Throughout the pectorals are main fac- 

 tors in propulsion, but in certain groups they are greatly aided by a caudal. All 

 Piatosomia have the absorbent surfaces in the intestine much increased by means 

 of spiral folds. The majority are ovoviviparous ; the Raiidae are oviparous. 



Body, including the head and the expanded pectorals, discoid. Eyes and 

 spiracles superior. Mouth inferior, more or less protrusible. Gill openings 

 inferior. Teeth in bands or pavements, compressed or cuspidate to broad and 

 plate-like. Nasoral grooves present or absent. Electric organs absent or pres- 

 ent. No anal fin. Tail short to long and whip-Uke; with or without dorsal 

 fins, a caudal fin or a serrated caudal spine. Pectorals with elongate basal and 

 numerous radial cartilages, extending forward above the gill openings. 



Synopsis of Groups of Families. 



Bodj^ and pectorals depressed to form a disk, narrow and elongate, short 

 to broad; skull produced in a median rostral cartilage, long to short; tail strong, 

 with two dorsals and a well-developed caudal; electric organs absent or incipient; 

 copula of hypobranchial cartilages unsegmented; nasoral grooves, from nos- 

 trils to mouth, absent or incipient; teeth small, in pavement; pelvis transverse: 



Rhinohatoidei (page 258) 



Pectorals including electric organs beside the body in a broad rounded disk; 

 skull produced in a short stout branching median rostral cartilage; tail short, 

 with two dorsals, one dorsal, or none, and a well-developed caudal; copula 

 incomplete; nasoral grooves; teeth small, in bands, raptorial; pelvis arched 

 backward, with lateral prepelvic processes: . . Narcoidei (page 259) 



Pectorals and body broad, angular to rounded, without electric organs; 



