ll'.tl 



not iiiK-oimiioii, are l)icus|ii(l instcail of ti'ieiDsiiid — tlici-c 

 not i.m\\ appear anterior suetorial teetli {ist, above in 

 tile tiaure), in almost tlie same arraiigenient as tiiose of 

 tiie Sea l.anipreA" and witli lozenged frauU'S, liiit tlie 

 male also possesses, on tiie posterior part of the disk, 

 a cur\'ed tliougli irregular row of teetli (/,s/, lieiow in 

 the figure), fhe speeitie eharaeter in the anterior dental 

 plate of the luoutii {at) we ha\-e pointed out abo\'e. The 

 posterior dental jjlate {jit) is of the same form as in tiie 

 preeeding speeies; hut its two outermost cusps are larg- 

 er than the others and oftenest bifid (bicuspid). The 

 number of cus]js between theni is as a rule ti\e, exeep- 

 tioiiall)' (fig. 353) four, so that the total number of cusps 

 varies between 7 and 9. The lateral dental plates of 

 the mouth {(tit, mlt, and y*//) are similar to those of the 

 i)receding speeies, but lie farther apart, and the middle 

 pair on each side {mlt) are, as we have mentioned, 

 usually tricuspid. The anterior linguodental plate {aJn) 

 is crescent-shaped, with the median cusp on the uni- 

 formh- curved margin largest, and with h or (i small 

 cusps on the lateral parts. The posterior linguodental 

 plates lie, when at rest, deej) in a groove between the 

 tumid lateral halves of the tongue", and are only slightly 

 curved but otherwise of the same form as in the pre- 

 ceding species. Their 10 or 12 retral teeth are very small. 

 The orifice of the nasal duct undergoes almost the 

 same alterations as in the preceding species. In the 

 Pride it is pear-shaped, anteriorly pointed, with the 

 dermal margin more elevated and expanded behind. In 

 the Lampern it is more uniformly I'ound, with the mar- 

 gin more or less elevated. In consequence of the some- 

 what shorter snout, the distance between this orifice and 

 the tip of the former' is also less than in the Sea Lam- 

 prey. In the Pride this distance is rather less than "5 

 (about 37 '/2 *)i ill the Lampern rather less than ^;^ 

 (about 53—59 %'), of the length of the head to the first 

 gill-opening. The series of gill-openings is somewhat 



longer than in the preceding species, varying, however, 

 between about H' ,, and 11' ., ?^ of the length of the 

 body; and tlu; st;veral gill-oijenings maj' be closed from 

 the anterior margin by a continuous, even flaj) of skin. 

 In tlie Lampern too their posterior margin is verruculose. 

 In the form and jiosition of the tins we can indeed 

 distinguish between two varicities of Lampern, but only 

 with far from reliable characters; and the variability in 

 these relations is such that we cannot even state a per- 

 fectly trustworthy distinrtiuii between the Lampern and 

 the Sea Lamprey. The rule, however, is that the limit 

 between the caudal fin and the second dorsal is less 

 distinct in the Lampern; and as the two dorsal fins are 

 sometimes contiguous, and a fin-like, though rayless, 



^#^' 



Fig. 353. Oral disk of a iimlc- Lain|ic-rn {Petromyzon JluL-itttilis, v;ir.), 



31' o cm. long from Arcli!iiigc-1, X 2. Signification of tlie letter« 



explaincil in llie preceding figure. 



dermal ridge is usually perceptible between the caudal 

 tin and the anus, the Lampern may be regarded as a 

 loAver form, with more distinct vestiges of the larval 

 condition. As regards the difference between the two 

 assumed varieties of Lampern, it has been asserted, first 

 and foremost, that the exclusively fresh-water form 

 Petromyzon hrdiichidlis or Planeri in general has higher 

 and closelj' contiguous dorsal fins. According to Kro- 

 YER the following relations hold good: 



Length ut' the Ijody expressed in millimetres 



Greatest heiiiht iif the first dorsal tin in % of the total length of the head and hranchial region 



., ., .. „ ,. ,. „ „ „ ,. „ distance between the tirst dorsal and the tip of the snout . 



„ „ „ „ second „ „ „ „ „ „ total length of the head and branchial region 



„ „ ,. „ „ ,, „ „ „ „ ,. distance between the first ilorsal and the tip of the snout.. 



" Our figure (PI. LIII. fig. 2) of tlie oral disk, as it appeared through the glass wall of an aquarium, where the Lampern had at- 

 taclicd itself, shows in the middle these lateral halves, separated by a V-shaped groove under the anterior dental plate of the mouth. Under 

 them the anterior linguodental plate, which is also white, may be seen; lint the dental cusps are too minute to be visible in the figure. 



'' In the preceding species about 63 — 68 %. 



X,-n,„i:„n,-inn Fishes 150 



