Larval Sfaf/es- of Triloliifc.s. — Beechrr. 191 
The histoiy of the acquired characters is traced by nieane 
of comparisons between hirval and adult trilobites. through 
paleozoic time, and a progressive series of larval forms estab- 
lished in exact correlation with adult forms, which them- 
selves constitute a progressive series, chronologically and 
structurally. 
The antiquity of trilobites is indicated by their remains in 
the oldest Paleozoic rocks, and especially by the fact that in 
the early Cambrian they are already much specialized aiul dif- 
ferentiated in number of genera. The age of the trilobite or 
crustacean phylum is further shown from the distinct larval 
stages of trilobites and their having a head and pygidiuni of 
consolidated segments. 
Since the trilobites are among tlie oldest and most general- 
ized of Crustacea, their ontogeny is of considerable im))ort- 
ance in interpreting crustacean phylogeny. 
The protaspis in its segmentation shows that th(^ ccplialDU 
had five pairs of appendages as in the adult. 
The crustacean nauplius is shown to be Jiomologous with 
the protaspis and to have potentially live cephalic segments 
bearing appendages, which shf)uld thereff)re be taken as char- 
acteristic of a protonauplius. 
The nauplius is a moditietl crustacean larva. Tlio protaspis 
more nearl}"- represents the primitive ancestral larval form for 
the class, and approximates the protonaujilius. 
IX. Keferkncks. 
1. Balfour, F. M., 188.').— A Treatise on Comparative Einhiyolosy, Me- 
morial edition. 
2. Barrande, J., 1849.— ,S'«o liirsiita Barrande, eiu Bruchstuck aus deni 
"System siluriendu centre de la Boheme."" Xeues Jahrl).fiir Min., 
Geol., etc. 
3. 1852. — Systeuie silurieu du centre de la Boheme. Ir partie. 
4. Beecher, C. E., 1893. Larval form.s of Trilol)ites from the Lower 
Helderberg Group. Am. Jour. Sci., Ill, vol. xi.vi. 
5. 1893,— A Larval Form of Triarthrus. Am. Jour. Sci., IH, vol. 
XLVI. 
(j. 1893. On the Thoracic Legs of Triiirtlunis. Am. Jour. Sci., 
Ill, vol. XLVI. 
7. 1894. -On the Mode of Occurrence, and the Structure and De- 
velopment of Triarthrus Becki. Am. Gkoi.ocist, vol. xiii. 
1894. — The AxJpenda^es of the Py^idium of Triarthrus. Am. 
Jour. Sci., vol. xLvii. 
9. 1895. — Further Observations on the Ventral Structure of Tri- 
arthrus. Am. Geolooist. vol. xv. 
