618 ARTHROPODA SUB-KINGDOM VII 
head portion predominating ; axis of cranidium with five annulations ; abdominal 
portion usually less than one-third the length of the shield ; axis with from one to 
several annulations ; pleural portion smooth or grooved , eyes, when present, anterior 
marginal, or submarginal ; free cheeks, when visible, narrow and marginal. 
The changes taking place during the growth of an individual are chiefly 
the following :—Elongation of the body through the gradual addition of the 
free thoracic segments ; translation of the eyes, when present ; modifications 
in the glabella ; growth of the free cheeks ; and final assumption of the mature 
specific characters of pygidium and ornamentation. 
In a classification of the stages of growth, the protaspis has the rank of a 
phylembryo, and corresponds in value to the protoconch of Cephalopods, the 
prodissoconch of Pelecypods, and the protegulum of Brachiopods. In its 
geological history and the metamorphoses it undergoes to produce the perfect 
Trilobite, accurate information can be gained as to what the primitive char- 
acters are, and the relative values of other features acquired during the long 
existence of the class. 
Of the stages of growth after the protaspis, the nepionic may be considered 
as including the animal when the cephalon and pygidium are distinct, and the 
A B 
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Fie. 1272 Fic. 1273. Fic. 1274. 
Ptychoparia Kingi, Meek. Sao hirsuta, Barr. Cambrian. Triarthrus Becki, Green. 
Cambrian. A, Protaspis en- A, Protaspis enlarged. B, Adult Ordovician. A, Protaspis en- 
larged. BL, Adult reduced. reduced. larged. B, Adult reduced. 
thorax incomplete. There would thus be as many nepionic stages as there 
are thoracic segments. The neanic stages would be represented by the 
animal with all parts complete, but with the average growth incomplete. 



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Fia. 1275. Fic. 1276. Fic, 1277. 
Provtus parviusculus, Hall. Acidaspis tuberculata, Dulmanites socialis, Barr. 
Ordovician. A,  Protaspis Conrad. Devonian. A, Pro- Ordovician. A, Protaspis en- 
much enlarged. £6, Adult taspis enlarged. B, Adult larged. — By Adult reduced. 
slightly enlarged. reduced. (Figs. 1272-77 after Beecher.) 
Final progressive growth and development of the individual would fall under 
the ephebic stage. Lastly, general evidences of senility would be interpreted 
as belonging to the gerontic stage. 
Morphogeny.—During the protaspis stage, several moults take place before 
the complete separation of the pygidium or the introduction of thoracic seg- 
ments. These bring about various changes, namely, the stronger annulation 
of the axis, the appearance of the free cheeks on the dorsal side, and develop- 
ment of the pygidium by the introduction of new appendages and segments, 
in la 
