U6 21ie American Geolocjist February, 1895 
limb towards the axial line, forming a gnathobase, and the 
progressive development of this member. First it has a slen- 
der cylindrical form in the posterior lialf of the series, then 
becomes flattened and denticulate, and finally widens, vintil 
on the head it forms the triangular plate-like coxopodite, with 
masticatory ridge and functioning as a gnathite (plate iv, 
figure 1; plate v, figures 1-4, 8-11). 
The large basal portions of the limbs of Aaaplnis^ in the 
"specimen illustrated b}' A¥alcott (Science, March, 1884), are 
evidently the gnathobases, as will be seen at once from a 
comparison with Triarfhrus (plate iv, figure 1). 
Oiujans in the median line. 
The hjipostoina. There is nothing peculiar in the hypos- 
toma of Triarthras, since it presents features commonly found 
in many other genera. It is longer than wide, and extends 
more than half the length of the head. The posterior end is 
narrowly rounded, margined by a slight doublure, and often 
presents a transverse elevation near the apex, as shown on 
plate V, figure 9. This may represent a corresponding hollow 
on the inner side to allow for movements of the numducatory 
organs. 
In considering tlie exact location of the appendages of the 
head, it must be understood that in their present positions 
they are probably somewhat displaced. During the process 
of fossilization th(} whole inner tissues of the animal were re- 
moved without replacement, allowing the ventral membrane 
to come more or less in contact with the under side of the 
dorsal crust, and thus causing some stretching of the mem- 
brane and consequent displacement of the organs. The hypos- 
toma, being more rigid and attached in front to the margin of 
the head, doubtless was not shifted but dropped down into 
the cavity of the glabella. When raised to its natural posi- 
tion it probably extended a little over the mouth parts. The 
fact that the mouth and lower lip do not come opposite the 
organs correlated as mandibles may be due in part to the un- 
equal stretching of the membrane over the uneven inner sur- 
face of the dorsal crust. 'JMie extended giuithobases directed 
obliquely backward and lying in the axial hollow cause the 
appendages to appear as though originating further back than 
is reallv the case. Nevertheless, the post(n-ior position of the 
