THE TRILOBITA. 



237 



M. Barrande ^ has succeeded in tracing out the develop- 

 ment of some species of Trilobites. He finds that the small- 



FiG. 57.— Diagram of Balanites (after Pictet).— ^, head ; 1, marginal band ; 2, mar- 

 ginal groove, internal to the baud ; 3, occipital segment ; 4, glahellum ; 5, great 

 suture ; 6. eyes ; a, fixed gena ; b, separable gena ; g, genal angle ; B, thorax ; 7, 

 axis or tergum ; 8, pleuron ; C, pygidium ; 9, tergal ; 10, pleural portions of the 

 pygidium. 



est, and therefore the youngest, forms are discoidal bodies, 

 without any clear evidence of segmentation. The division 

 into somites takes place by degrees, the number increasing 

 up to the adult condition. It is possible that still younger 

 conditions may have escaped fossilization, but the analogy 

 of Limulus suggests that these small discoidal forms really 

 represent the condition in which the Trilobite left the egg. 



The Merostomata.'' — The only existing representative of 

 this division of the Crustacea is the genus Limulus (the King 

 Crabs or Horseshoe Crabs), the various species of which are 



^ " Svsteme Silurian du centre de Bolieme." tome i. Trilobites. 1852. 

 2 H. Woodward, " A Monograph of the British Fossil Crustacea belonging 

 to the Order Merostomata," 1866. 



