176 The American Geologist. Septembor, 1)S95 
ture of the youngest stages observed, the elementary forms of 
this genus are as yet unknown, and possibly the extreme tenu- 
ity of the test in the protaspis has prevented their preserva- 
tion. In the same way the young of Trinuclens (figure 3) 
show a separate cephalon and pygidium, and the specimens 
are in a much more advanced stage of development than the 
protaspis of Pro'ctus, shown on plate IX, figure 5. An evi- 
dence of age is furnished, also, in the transverse shape of the 
head, which, in typical elementar}'- forms, is longer than wide, 
instead of wider than long. 
The youngest specimens of Areflmsind koitinrki, figured by 
Rarrande,-^ are 2 mm. or upwards in length and have seven or 
more free thoracic segments, with the cephalon wider than 
long. The facts of ontogeny show that j'^o Linger stages must 
be admitted in which the number of segments diminishes to 
nothing, continuing down to a form agreeing with the pro- 
taspis of other genera. 
It has already been suggested^ that the species described by 
Barrande'^ under the generic name of HydrocephoJiis< are prob- 
ably the young of Paradoxides. This conclusion receives fur- 
ther support from the undoubted young of Olene/hts. a related 
genus, which in its immature stages bears a strong resem- 
blance to Hydrocephalus. The j^oungest examples of the lat- 
ter have a distinct pygidium, a well-developed cephalon, and 
large eye-lobes at the sides of the glabella, as in adult forms. 
Free-cheeks were evidently present though not generally pre- 
served. See figures 4 and 5. 
The young of Olenellus asaphoide.s, described and illustrated 
by Ford" and Walcott,-^''' -^ also present a number of features 
considerably in advance of a typical protaspis. The imma- 
ture characters are mainly the large size of the cephalon and 
the distinct annulation of the axis. The post-protaspidian 
characters are the distinct and separate pygidium, the adult 
position of the eyes, and the apparent!}^ well-developed free- 
cheeks. In figure 7, after Ford,— the outer pair of spines be- 
longs to the free-cheeks, the other pair being formed by the 
])leural extensions of the glabella, which were called the in- 
terocular spines. See also figures 6 and 8. 
The young specimen of Ptychoparia man He Salter sp., fig- 
ured and noticed by Callaway,i'Ms 1.5 mm. in length, and 
