SÜBCLASS I 



TRILOBITA 



703 



imbricate, and their fulcra are provided with facets upon which the fulcra of 

 adjacent segments impinge. The ends of the pleura thus protect the ventral 

 surface along the sides, when the animal is enrolled. Some forms appear to 

 have possessed the power only to a limited degree. In these, the creature is 

 usually found extended, and the facets on the fulcra are either rudimentary 

 or absent. 



Ontogeny. — Minute spherical or ovoid fossils associated with Trilobites 

 have been described as possible Trilobite eggs, but nothing is known, of 

 course, of the embryonic stages of the animals themselves. The smallest and 

 most primitive organisms which have been detected, and traced by means of a 

 series of specimens through successive changes into adult Trilobites, are little 

 discoid or ovate bodies not more than 1 mm. in length. This first larval form 

 has been named the protaspis, and has been found to be the typical larval form 

 characteristic of all Trilobites. It is believed to approximate the protonauplius 

 form, or the theoretical, primitive, ancestral, larval form of the Crustacea. 



The simple characters possessed by the protaspis are the following, as 

 drawn from the study of this stage in all the principal groups of Trilobites : — 

 Dorsal shield minute, not more than 0*4 to 1 mm. in length ; circular or ovate in 

 form ; axis disti7ict, more or less strongly annulated, limited by longitudinal grooves ; 

 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 ; development of the pygidium ; 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 development, 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 developmental stages after the protaspis, the nepionic may be con- 



Fio. 1348. 



Ptychoparia Jdngi Moek. 

 Cambriaii. A, Protaspis en- 

 lar^Rfl. B, Adult reduced. 



Fia. 1349. 



Sao hirsuta Barr. Cambrian. 

 ^, Protaspis enlarged. ü, Adult 

 reduced. 



Fio. 1350. 



Triarthrus becki Green. 

 Ordovician. A, Protaspis en- 

 larged. B, Adult reduced. 



sidered as including the animal when the cephalon and pygidium are distinct, 

 and the 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. 



