28 
William Patten. 
Arthropods a group of animais, nearly all of which liave osseous, dermal skeletons, and 
some of which have such characteristically vertehrgte bodies and fins as the Gephalaspidae 
and Asterolepidae \ or to include in the vertebrates, animais having many pairs of jointed 
appendages, it becomes necessary to create for them a new dass, one that shall occupy a 
position between the true vertebrates and Arthropods, and unité these two great groups 
into one compact pliyum. 
I propose, for this new dass, the name Peltocephalata, and include in it forms like 
Pterichthys, Cephalaspis, Pteraspis , Tremataspis , and related genera. The dass may be pro- 
visionally characterized as follows. 
L. The Peltocephalata were arthropod-like animais, raoviug about through the soft 
mud on the bottom of shallow waters, in the typical Arthropod position. Most of the body 
was probably concealed, leaving only the prominent median eyes exposed. The presence of 
paired oar-like appendages indicates the power of free swimming, but the more or less rigid 
and clumsy appendages, and heavily armored body, could have produced little more than 
brief, spasmodic excursions, like those of adult Limuli and Eurypterids, or jerky, intermit¬ 
tent flights through the water, like those of a Copepod. And, just as in these examples, the 
shape of the body and the position of the appendages in reference to the centre of gravity 
compel the free swimming individual to reverse the usual position of dorsal and ventral sur¬ 
faces, so in the Peltocephalata the prevalence of the same conditions must have forced them, 
after leaving the bottom, to turn over and swim with the neural side uppermost, in the true 
vertebrate position. The swimming movements were aided in some cases by numerous small 
appendages on the head and trunk, and fishlike caudal fins and tail were also used in swim¬ 
ming and in reversing the position of the dorsal and ventral surfaces. 
It was not tili this new method of locomotion had completely replaced the old, that 
the eyes left the liaemel surface (their position in most adult Arthropods) and returned to 
the neural surface of the body, (their position in enibryo Arthropods and their permanent 
position in Vertebrates). 
The Exoskeleton was a true dermal armor of eetodermic origin, intermediate between 
the type presented by Limulus and that of the more modern Vertebrates. It consisted of 
three principal layers ; the middle one containing large, more or less regulär spaces or can- 
cellae. The matrix was strongly laminated and penetrated by numerous dentine-like tubul- 
es, or pore canals, and contained eitlier unipolar or multipolar osseous lacunae. The trunk 
was covered with rhomboidal scales or with segmentally arranged ring-like plates. The 
presence of a System of lateral line Organs is indicated by numerous, pit-like markings ar¬ 
ranged in linear séries. 
A flattened cartilagenous cranium was present, but notochord and vertebral arches 
were absent or rudimentary. Median and lateral eyes were enclosed in bony orbits, some- 
times protected by hard, convex lens-like coverings continuous with the outer layers of the 
