THE NATURAL HISTORY SURVEY. 169 



under to form a reflexed margin or doublure. This doublure 

 is separated from the dorsal test by a narrow space, and pro- 

 duces the hollow spines at the ends of the thoracic segments as 

 well as the genal spines when they are present, also the infolded 

 margin of the cephalon and the pygidium. Among some 

 species of trilobites two forms have been observed, one broader 

 and larger which has been considered to be the male, and a 

 smaller and narrower form believed to be the female. Still 

 another supposed sexual difference has been observed in at 

 least one species of Dalmanites^ where the female form has a 

 more ancestral type of glabella and rounded genal angles, the 

 male exhibiting more progressive characters in the glabella 

 and well developed genal spines. 



The cephalon or head of the trilobite comprises the anterior 

 one of the three transverse regions of the carapace, and is sup- 

 posed to have originated through the coalescence of several 

 segments in some unknown ancestral form. It is crescentiform, 

 semicircular, semielliptical, or sub -triangular in form, ' and is 

 sometimes greatly produced in front. Its postero-lateral angles 

 are known as the genal angles and are often produced into spines 

 of greater or less length, the genal spines. The axial portion of 

 the head, bounded by the axial or dorsal furrows, is more or less 

 sharply differentiated from the general surface and is called the 

 glabella. The glabella is exceedingly variable in size and form; 

 in its primitive condition it is divided by transverse furrows 

 into five lobes which are supposed to represent the original five 

 segments which have been consolidated. In many genera the 

 primitive pentamerous lobation of the glabella has been nearly 

 or wholly obscured by the modification or elimination of the 

 transverse furrows. The five primitive lobes of the glabella 

 are designated, beginning with the anterior one, the frontal 

 lobe, -the first, second and third pairs of lateral lobes, and the 

 occipital segment or lobe. With the exception of the posterior 

 one, the four transverse furrows of the primitive glabella are 

 usually not continuous across the median portion of the glabella, 

 they are designated, beginning anteriorly, the first, second and 

 third lateral glabellar furrows, and the occipital furrow. The 

 occipital furrow is always retained whatever may be the modi- 

 fication of the lateral furrows. 



The pleural regions of the head or cephalon are called cheeks. 

 In the great majority of trilobites each cheek is divided into 



*G. van Ingen, School of Mines Quart., Vol. 23, p. 69 (1901). 



