DISCUSSION OP SPECIES AND THEIR DISTRIBUTION. 5 



The color is dark, except iipou the belly, which is grayish. The second dorsal is darker 

 across the middle in front and towards the margin behind, while the candal is blackish 

 throughout. 



MEASUREMENTS. 



Inches. 



Total length 1 1 



Snout to eye 1 



Snout to first bramhial a]i('iture ._ li'^- 



Snout to seveutU branchial aperture 2^^ 



Snout to commencement of tirst dorsal 5f 



Snout to end of first dorsal 64 



Snout to commencement of second dorsal ^^\ 



Snout to end of second dorsal 10 



Snout to end of (-audal 11 



Snout to counuencement of caudal below 9\^ 



Snout to corameuccmeut of caudal above 10 



Snout to anus 7f 



This hitherto undescrilMMl form is evidently most closely related to the tj^iical species 

 of the genus retromyzon^ but differs by the obsolescence of the armatiire of the suproral 

 and infroral laminae, while differences of proportion characterize the species; it is scarcely 

 generically distinct from retrnmyzon, but may be distinguished as a subgeiieric type under 

 the name Bnthymyzon witli the following characters: 



BATHYMYZON. 



Petromyzontinw with the sujiroral lamina contracted, its two converging teeth almost 

 completely fused together and only evident at the summit of the combined m.ass, infroral 

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

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

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

 with the cusps approxinr'ited, 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 inner lateral angles; the anterior dorsal 

 tin 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 tins, the posterior pair 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 

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

 bulb with three series of valves; intestiues 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. (;ill opcuiugs, 5-7, slit-liUc; jaws detached from skull Ski.achii (Sharks and Rays) 



II. Hill openings single, four branchial clefts; jaw and palate attacLed to skull.. . HoloCepham (Chimieras) 



The class ElaxmohrKHchii, intermediate between the true tishes and the Marsipobranchi- 

 ates, is sparingly represented in the abyss;il faunas. 



Subclass SELACHII. 



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

 slit-like, tive {sometimes six or seven) in number, sometimes lateral or inferior; jaws de- 

 tached from the skull; opercular and pelvic bones lacking. 



