1184 
SCANDINAVIAN FISHES. 
elliptical disk. In its first-mentioned condition, when 
the disk can almost be folded together from the sides, 
its length is about equal to the depth of the body, 
somewhat greater or less, and as a rule more than 4 / 5 
of the length of the snout (to the anterior margin of 
the eyes), which varies between 10 a and 8 % (excep- 
tionally approaching 7 %) of the length of the body 
or between 2 / s b and 3 / 4 of that of the head to the first 
gill-opening. 
In the mere size of this suctorial disk, even rela- 
tively greater than in the following forms, we have a 
est 
median line), directed in a more or less sharp crook 
towards the mouth; but exactly in the preoral median 
line the row is straight, and the rows on the posterior 
part of the suctorial disk are only slightly curved, each 
row being directed inwards with fair regularity towards 
one of the teeth (cusps) of the posterior dental plate 
of the mouth. The teeth in these rows (the suctorial 
teeth, tig. 352, est and ist ) are unicuspidate, conical, 
gradually increasing in size inwards, separated by 
grooves in the gums, which are thus divided into lo- 
zenges, one to each tooth. Four rows meet the sides 
Fig. 352. Oral disk of a Lamprey ( Petromyzon marinus) 8Y 2 cm. long, X 2. 
aln, anterior linguodental plate; alt, anterior lateral dental plate; at, anterior dental plate; est, outer suctorial teeth; ist, inner suctorial teeth ; 
mil, mediolateral dental plate; pin, posterior linguodental plate; pit, posterior lateral dental plate; pt, posterior dental plate. 
very good character for the Sea Lamprey; but the most 
characteristic peculiarities are shown by the dentition 
of the mouth and tongue. The suctorial disk is edged 
with a double or multifarious series of densely set 
fringes (dermal papillie), and within these appear nu- 
merous rows of teeth (12 or 11 on each side of the 
of the mouth, and the two innermost (largest) teeth in 
these rows (the lateral teeth, alt, mlt, and pit) are so 
closely set that they are joined at the base into bicuspid 
teeth, each on its own root (lateral dental plate). The 
bicuspid anterior dental plate of the mouth (at), situated 
between but a little behind the anterior lateral plates, 
“ Sometimes 11 in the variety occurring in Cayuga Lake (at Ithaca, N. Y.) ; see Meek in Jordan and Fordice, 1. c. 
b In exceptional cases only 63 see Meek, 1. c. 
