292 AN INTRODUCTION TO THE STUDY OF FOSSILS 



— the cornea, as in Calymene ; or (2) each facet with a separate 

 cornea, as in Phacops (Fig. 131, C). 



Preservation. — Since the thin ventral membrane and deli- 

 cate limbs were usually quickly destroyed after the molting or 

 the death of an animal, they are very seldom preserved in the 

 fossil state ; and as sediment would cling readily to the angu- 

 lar ventral surface of the dorsal shield, the hardened sediment 

 splits most easily along the comparatively smooth dorsal surface. 

 Hence trilobites are almost always found as mere exteriors of 

 the dorsal shield, and even this shield occurs comparatively 

 seldom as a complete whole, for its parts are easily separable 

 from one another. 



Development. — Judging by existing Crustacea, reproduc- 

 tion among the trilobites was sexual. Frequently the same 

 species shows a broad and a narrow form ; the former may repre- 

 sent the female and the latter the male. The minute rounded 

 bodies associated with many trilobite remains are regarded 

 as possible trilobite eggs. The earliest stage in their develop- 

 ment definitely known is the larval form, — named protaspis. 

 This is believed to approach closely the proto-nauplius, the 

 theoretical primitive larval form of all Crustacea, occurring as 

 it does in the Lower Cambrian and possessed of such simple 

 characters. The dorsal shield of this protaspis has been dis- 

 covered in all the principal trilobite groups and its characters 

 may be summed up as follows : minute, less than i mm. long, 

 circular or ovate in outline, with distinct and more or less strongly 

 annulated axial lobe. It consists of a large cephalon and a very 

 small pygidium. The glabella has at the beginning, or after a few 

 molts, five transverse grooves ; eyes when present are antero- 

 marginal or submarginal; free cheeks when visible are narrow 

 and marginal. The pygidium has an axial lobe of one to 

 several annulations, its pleural portion smooth or grooved 

 (Fig. 126 E, F). 



The various stages in the growth of a trilobite have been studied 

 by means of long series of specimens, separate molts of the same 



