DISCUSSION OF SPECIES AND THEIR DISTRIBUTION. 5 



The color is dark, except npoii tlie belly, which is grayish. The second dorsal is darker 

 across the middle iu front and towards the margin behind, while the caudal is blackish 

 throughout. 



MEASUREMENTS. 



Inches. 



Tiital length 11 



Snout to eye 1 



Snout to first branchial aperture 1 ^7^, 



Snout to seventh braiichial api>rture 2A 



Snout to oommencemeut of first dorsal 5j 



Snout to end of first dorsal 6J 



Snout to commencement of secfuul dorsal 7.,'^ 



Snout to end of second dorsal 10 



Snout to end of caudal 11 



Snout to commencement of caudal below 9j !. 



Snout to commencement of cauilal above 10 



Snout to anus 7| 



This hitherto undescribed form is evidently most closely related to the tyi)ical species 

 of the genus Petromyzon, but (litters l)y the obsolescence of the armature of the suproral 

 and infroral lamina', while dirt'erences of proportion characterize the species; it is scarcely 

 generically distinct ii'oin Petromi/zon, but may be distinguished as a subgeneric type under 

 the name Bathymyzon with the following characters: 



BATHYMYZON. 



Pctromyzontinw with the suproral lamina contracted, its two converging teeth almost 

 completely fused together and only evident at the summit of the combined mass, infroral 

 lamina crescentiform and spout-like at the middle, and with the denticles obsolete, discope- 

 ripheral teeth numerous and iu obliquely-arched series of 4-7, declining downwards; the 

 innermost lateral teeth of the four rows diverging from the mouth, in each side bicuspid, 

 with the cusps apiiroximated, and diminishing downwards rapidly; the lingual teeth 3, 

 pectinate, the anterior deeply impressed and sulcate backwards at the middle and the pos- 

 terior correspondingly curved backwards at their iauer lateral angles; the anterior dorsal 

 fin distinct from posterior. (Gill, MS.) 



Class ELASMOBRANCHII. 



Lyriferous vertebrates with cartilaginous skeleton, and destitute of membrane or der- 

 mal bones; no cranial sutures. Body with vertical and paired fins, the posterior jiair ab- 

 dominal; caudal tin with elongated upper lobe; gills attached by their outer edges to the 

 skin, with an intervening gill opening between each; no gill cover; skin naked or covered 

 with minute imbricated scales or hard plates, sometimes spinous; no air bladder; arterial 

 bulb with three series of valves; intestines with a spiral valve; optic nerves united, not 

 decussating, or only slightly so; ova few and large, fertilized, and sometimes developed 

 internally; embryo with external deciduous gills; males with intromittent organs attached 



to ventrals. 



KEY TO SUBCLASSES OF ELASMOBRANCHII. 



I. Oill oiieniugs, 5-7, slit-like ; .jaws detached from skull SelaCHII (Sharks and Rays) 



II. (iill openings single, four branchial clefts ; jaw and palate attached to skull . . . Holocephali (ChiniaTas) 



The class Elasmohranchli, intermediate between the true fishes and the Marsipobraiiciii- 

 ates, is sparingly represented iu the abyssal faunas. 



Subclass SELACHII. 



Elasmobranchiates with body more or less cylindrical or depressed, with gill openings 

 slit-like, five (sometimes six or seven) iu number, sometimes lateral or inferior; Jaws de- 

 tached from the skull ; opercular and pelvic bones lacking. 



