Larval Sta(/es of 2'rilobites. — Beecher. 167 
taken of all the known early larval stages thus far desci-ibed. 
This work would have no special interest in itself were it not 
for the fact, that, with our present understanding of trilobite 
morphology, it is possible to reach some conclusions of general 
importance, which have a direct bearing on the significance 
and interpretation of several of the leading features of the 
trilobite carapace, and incidentally upon the structure and re- 
lations of the nauplius of the higher Crustacea. 
II. The Protaspis. 
Barrande"^ recognized four orders of development in the 
trilobites, as follows : 
( Head predominating, incomplete. ^ Types. 
I. \ Thorax nothing or rudimentary. I Sao hirsuta. 
( Pygidium nothing. ) 
i, Head distinct, incomplete. J Trinucleus ornatus, and 
II. Thorax nothing. , , all Agnostus. 
{ Pygidium distinct, incomplete. ) ^ 
( Head complete. ) 
III. ] Thorax distinct, incomplete. [• Arethiisina konincki. 
( Pygidium distinct, incomplete. ; 
( Head complete. ') * 
IV. i Thorax complete. [- Dalmanites Jiaiismanni. 
I Pygidium distinct, incomplete. ) 
A study of these groups shows at once that they form a 
progressive series in which the first alone is primitive. The 
others are more advanced stages of development, as shown by 
the larger size of the individuals, and their having characters 
which appear successively in the ontogeny of a species belong- 
ing to the first order of development. To attain the stage 
which is represented by actual specimens, they must have 
passed through earlier stages, which as yet have not been 
found. Furthermore, it is evident that Barrande did not con- 
sider the orders after the first as primitive, and characteristic 
of the genera cited, for, in some remarks under the third or- 
der, he says=^: "II est tres-vraisemblable, que la plupart des 
Trilobites de cette section, si ce n'est tons, devront etre un 
jour transferes dans la premiere, par suite de la decouverte 
probable d'embryons sans segmens thoracique." 
The geological conditions necessary for the fossilization of 
the minute larval forms of trilobites are such, that only in 
comparatively rare instances are any of the immature stages 
preserved. Larval specimens are doubtless often overlooked 
or neglected by collectors, but generally the sediments are too 
