224 
whether one or the other of these two genera is the more prim¬ 
itive, as neither the biology nor other features yield any foothold 
for a reliable conclusion in that respect. Both genera show the 
central characters of the group, viz. a naked peduncle, a carina 
with a basal umbo, and scuta with their umbones where the basal 
and the occludent margins meet. In both genera there is an evid¬ 
ent tendency towards a reduction of the formation of carbonate of 
lime in the plates, although this feature is a little more pro- 
nounced in Poecilasma than in Lepas. The two genera are closely re- 
lated, indeed so closely that Darwin (1852) made his excuses, 
when he introduced Poecilasma beside Lepas, and with these two 
forms the other genera with naked peduncles have originated. 
According to the theories of previous authors, the scalpelloid 
group starts from Oxynaspis (Darwin), or the forms with naked 
peduncles are derived from the scapelloid group (Hoek, Gruvel). 
The males of the primitive Scalpellidae contradict the first 
theory. The other theory is contradicted by other facts; no trace 
can be found in adult or juvenile specimens, or during the post- 
embryonic development of a peduncle skeleton in the Lepadid 
group, and every trace of accessory capitulum plates is also absent. 
Stronger weight must however be ascribed to the fundamentally dif- 
ferent position of the carinal, and scutal umbones in the groups. As 
long as the capability of lime secretion is mainly bound to the 
transition from capitulum to peduncle, the umbones of the five 
primary valves are apically situated; the formation of accessive 
plates (both plates of the capitulum, and the scales of the peduncle) 
is as the investigations have shown, confined to this zone of lime 
secretion. If now, as in Lepadid ae s. str., the capability of lime 
secretion moves away from the transition to the peduncle, and is 
confined to the middle part of the capitulum, the umbones of scuta 
and carina will become basal, and, moreover, the formation of ac¬ 
cessive lower capitulum plates and peduncular scales will be pre- 
cluded. These features indicate that the two groups, or families, 
viz. the Scalpellidae, and Lepadidae s. str. have arisen separ- 
ately from the ancestral form. 
From Lepas the genus Conchoderma has evidently arisen; both 
genera agree in their mouth parts, and show a great development 
of the filamentary appendages. A further reduction of the skeleton 
