NARKAWAV-RAVAKtM) : A Xkw Amkrican Cvill'I.K. ()03 



Comparison' wnit Oihkr Si'Kcif.s. 



The junior writer is glad to take this 0])portunity to correct an error 

 into which he was led by our lack of knowledge of the glabella of the 

 American forms of Cybclc. In describing the trilobites of the Chazy 

 formation (An.nai.s Car.\f.gif. Museum, ^'ol. 3, 1905, p. 362), 

 certain isolated glabella and free cheeks of a tribolite found on Val- 

 cour Island, and at Valcour, New York, were referred to the genus 

 Glaphiinis on account of their resemblance to similar parts of Gla- 

 pJiiirus pustiilattis. Now that the glabella of the American Cybclc is 

 known, it becomes evident that the glabella; described as Glaphuriis 

 primus belong to the genus Cybclc. At Valcour these glabellar occur 

 associated with the pygidia described under the name of Cybclc 7'al- 

 coiirciisis by Raymond, and it seems probable that the two jjarts be- 

 long to the same species. Glaplitinis primus was described liefore 

 Cybele valcoureiisis and that name must take precedence. The Chazy 

 form should then be known as Cybclc priuuj, and the name Cybele 

 valcourcusis should be eliminated. 



Cybele ella is closely related to Cybele prima of the (^hazy and is 

 very probably a direct descendant, the differences being such as would 

 be expected in an evolutionary series. In the cephalon, Cybele prima 

 differs from Cybele ella in retaining, faintly it is true, the glabellar 

 furrows, and, in the Chazy species, the pits which represent the inner 

 ends of the furrows are all connected, while in the species just de- 

 scribed, only the second and third pits are connected. There seems 

 to be a tendency in several families of trilobites, notably in the Asa- 

 phidse, for the primitive segmentation of the cephalon and ])ygidium 

 to become obscured, forming, as a result of the process, smooth 

 cephalic and abdominal shields. In these two species of Cybele, two 

 stages of a similar ]3rocess can be seen. In Cybele prima the outer 

 ends of the glabellar furrows are becoming faint, and the inner ends 

 are represented by pits. In Cybele ella the outer ends are entirely 

 eliminated, and only the pits at the inner ends remain, and these have 

 become smaller by the isolation of the first pair from the succeeding 

 ones. 



In the pygidia fewer changes have taken place. The first pair of 

 ribs on the pleura are stronger in Cybele ella than in the Chazy spe- 

 cies, and the furrows on the sides of the axial lobe are somewhat less 

 prominent. 



