526 FISHES CHAP. 
small polygonal plates. All the plates are superficially orna- 
mented by small stellate tubercles. The ventral armature (Fig. 
315) is similar to the dorsal. A large mental plate forms the 
hinder margin of the transverse slit-like mouth, the anterior 
limit of which is defined by the rostral plates already mentioned. 
Laterally may be seen a pair of small plates (a), each perforated 
by a small aperture, and prob- 
‘i ably indicating the position 
of some kind of sense-organ. 
Posteriorly there is a large 
median ventral plate, in rela- 
tion with a pair of anterior 
and a pair of posterior ven- 
tral plates. The areas between 
the larger plates are filled in 
by numerous small polygonal 
plates. It is possible that 
there is a single external 
branchial aperture on each 
side, near the postero-lateral 
angle of the shield and_be- 
hind the posterior ventro- 
lateral plate. The sole repre- 
sentative of the family is 
Drepanaspis gemiindenensis, 
from the Lower Devonian of 
Gemiinden in Rhenish Prussia. 
Large examples of this fossil 
must have exceeded two feet 
in length. 
Fam. 3. Psammosteidae. 
—To this family are referred 
Fic, 315.—Ventral surface of Drepanaspis certain dermal plates occur- 
(tail in profile). «@.v.d, Anterior ventro- rine. an AoMoronenmlinee frag- 
lateral plates; e./, external lateral ; m, toy 
mental plate ; m.v, mid-ventral ; p./, pos- mentary condition. in the Old 
tero-lateral ; p.v./, posterior ventro-lateral ; ; : ; 
r, rostral ; x, orbit or sensory plate. The Red Sandstone and Devonian 
mouth and the supposed cloacal aperture are formations of Great Britain 
indicated in black. (From Traquair, ) : f i 3 
and Russia. In their size 
and shape, and in their stellate tubercles, these have been com- 
pared to the dorsal, postero-lateral, and ventral plates of 
