1182 
SCANDINAVIAN FISHES. 
Genus PETROMYZON. 
The diphycercal caudal fin continuous above with the posterior part of the dorsal. Anterior dented plate of the 
mouth furnished only at each end with an elevated tubercle or pointed tooth. 
As we have mentioned above, in the Lampreys 
we cannot speak of real jaws or jaw-teeth; but within 
the suctorial disk of adult Lampreys are set several 
horny, pointed teeth or terete protuberances, charac- 
teristic of different genera and species, and to these 
teeth it has been hitherto customary to give such names 
as if they were set in real jaws. The oral aperture 
is first surrounded, as we have seen, by an annular 
cartilage (fig. 343, ann). This bears both on its an- 
terior (upper) and its posterior (lower) margin an hori- 
zontal dental plate furnished with more or less pointed 
teeth (fig. 352), which may lie called respectively an- 
terior teeth (at) and posterior teeth (pt). On each side 
of the annular cartilage there lie in the sucking-disk, 
next the mouth, three small loose cartilages, each set 
with 1 — 3 horny teeth (protuberances), which we entitle 
the lateral teeth of the mouth (alt, mlt, and pit). Out- 
side these the sucking-disk has within its fimbriated 
margin a number of scattered teeth, arranged in rings 
or in curved rows, suctorial teeth, as Gunther calls 
them (ist and est). Within the mouth itself, between 
the anterior and posterior dental plates, appear the 
lingual teeth, an anterior ( din ) and two posterior (pin) 
curved dental plates, parallel to the posterior dental 
plate of the mouth. It must be observed, however, 
that when the tongue is protruded to its utmost extent 
its posterior teeth project beyond the anterior ones, and 
probably serve as the most powerful rasping organ of 
the Lampreys. The dental character assigned above to 
the genus Petr omy son is intended to express the fact 
that the anterior dental plate of the mouth is neither 
bipartite, as in the Pacific genus Mordacia, with a 
group of three teeth on each part, nor tridentate, as 
in the genus Ichthyomyzon from the west coast of North 
America". The other character given above differen- 
tiates all these genera from Geotria, a Pacific genus, 
with the caudal fin separated from the posterior dorsal. 
A deep-sea genus, Bathymyzon , “with the suproral and 
infroral plates or lamina? destitute of odontoid tu- 
bercles,” was distinguished by Gill*' among the fishes 
taken by the American fishing-schooner Albatross at a 
depth of 520 fathoms within the area of the Gulf Stream 
in lat. 49° N. He remarks, hoAvever, that the only 
species known (Bathymyzon Bairdii) comes very near 
to Petromyzon marinus. 
The species of the genus Petromyzon that belong 
to the Scandinavian fauna are the same as are found 
in other parts of Europe; but considering the variabi- 
lity of form which here asserts itself, we distinguish 
only the following: 
A: Anterior dental plate of the mouth 
so short that its two teeth stand close 
together — Subgenus Petromyzon Petromyzon marinus. 
B: Teeth on the crescent-like anterior 
dental plate distant from each other 
— Subg. Ammoccetes Petromyzon fluviatilis. 
a, var. major: the two dorsal fins 
more or less distinct- 
ly divided from each 
other — Petromyzon 
fluviatilis. 
b , var. minor: the two dorsal fins 
more or less distinct- 
ly confluent — Pe- 
tromyzon branchialis. 
a According to Jordan and Fordice a median tooth sometimes occurs on the anterior dental disk in Petromyzon fluviatilis branchialis , 
so that the genus Ichthyomyzon can hardly be retained. 
6 Proc. U. S. Nat. Mus. 1883, p. 254. 
