UNIO. 479 
the Cape Colony, Tasmania, and Patagonia. In Central Europe Unio reaches no higher 
than the great lakes at the foot of the Alps. It has a few representatives on the 
elevated plateau of Mexico, but is wanting, so far as we know, in the high central 
parts of Guatemala and Costa Rica; it is common, in various subgenera, on the Atlantic 
slope, less so on the Pacific slope, probably because the former has larger rivers and 
more low lands. 
H. v. Ihering has discovered that in all the South-American species which he has 
been able to examine the eggs are hatched within the inner gills, whereas it is well 
known that in the European and North-American forms this is done in the outer gills 
[see Archiv fiir Naturg. lix. p. 47 (1893)]. It is therefore very desirable that the eggs 
of living specimens of the Mexican and Central-American forms should be examined, 
and also to ascertain whether the young ones, after having left the gills of the mother, 
attach themselves to the fins of fishes by a byssal filament, as has been stated to be 
the case with European species. Hitherto no structural character has been found to 
distinguish the South-American forms from the rest of the genus; the summits of most 
of them are radiately sculptured, but this is also the case in many Indian species, the 
group of Unio corrugatus, Miill. 
The nearly allied genus Margaritana, Schumacher (without posterior lamelliform 
teeth), in its restricted sense palarctic and nearctic, but represented in South America 
by Monocondylea, d’Orb., and in the Malayan region by Pseudodon, Gould, is altogether 
wanting in Mexico and Central America. 
‘The species of the genus Unio are very numerous and many subgenera have been 
proposed by various authors for their reception. Fischer and Crosse admit for Mexico 
and Guatemala alone (Salvador, Nicaragua, Costa Rica, and Panama excluded) nineteen 
subgeneric sections, most of them first proposed by themselves without regard to foreign 
forms, and containing altogether fifty-six species. I shall follow their classification 
with some modifications. Eleven subgenera are here adopted, and in the characters 
and distribution of each given below I have endeavoured to show the relationship of 
the Mexican and Central-American forms with those of North or South America. 
I. Crenoponra *, Schliiter, Verz. meiner Conch. p. 33 (part.) (Halle, 1836) (restricted by Mérch, 
Cat. Yoldi, 1853, p. 45) : Plectomerus, Conrad, Proc. Acad. Phil. vi. p. 260 (1853) ; Fischer 
& Crosse, Miss. Scient. Mex., Moll. il. p. 555. no. 3 (1893) : Amblema, Rafinesque (1820) 
(part.). Non—Symphynote, plicate, quadrate Uniones, Lea, Synops. fam. Naiad. ed. 1, p. 12 
(1836), ed. 3, p. 20.—Shell with strong plaits, radiating from the summits, the strongest 
running obliquely to behind and below; very solid and large-sized ; eardinal teeth strong. 
Several species in North America, especially in the Ohio and neighbouring rivers. 
U. nicklinianus in N.E. Mexico; U. digitatus and U. stolli in Guatemala. Wanting in 
* Fischer, Manuel de Conch. p. 1000 (1885), quotes Unio securis as type of Crenodonta, Schliiter ; but this 
is only the third species in Schliiter’s list, plicatus being the first, and, moreover, Morch restricted the name to 
U. plicatus in 1853. 
