Trenton conglomerate of rtsedorph hill 61 



identity of these forms with I s o t e 1 u s g i g a s . As long 

 as the development and stages of Isotelus are not fully 

 known, it seems eminently proper to denote the presence of 

 these remains of immature character by the first specific name 

 proposed for them, and by the subgeneric term which has beeu 

 originated to include forms of the descending line of the Asa- 

 phidae, exactly corresponding with the generally recognized 

 subgenera Ptychopyge and Niobe of Angelin in the ascending 

 series of the Aisaphidae. (Group 6) 



illaenus Dalman 



Illaenus americanus Billings. Can. nat. and geol. 1859. 4:371 



Cranidia and pygidia of this species are frequently found in 

 the gray crystalline and the black compact limestone. They 

 range in size from 1 mm in length to the normal size of 

 adults. Most of them show the characteristic even convexity of 

 the glabella of this species (an exception is made for several 

 large cranidia from the pebbles with Tretaspis, in which the 

 posterior half is nearly flat, and the anterior part consequently 

 more abruptly bent downward) the sigmoidal curve of the dor- 

 sal furrows and the fine squamous striae. In the proportions of 

 the cranidia there exist wider differences, as was supposed by 

 Billings, and it almost seems as if there were two series of heads, 

 one wide and short, such as Billings figured, and one relatively 

 narrow and long, the younger heads specially showing frequently 

 the latter proportions. It is quite probable that these differ- 

 ences are of a sexual nature. Young specimens have the dor- 

 sal furrow extending longer, in the smallest examples to fully 

 the middle of the cranidium, whence a shallower depression 

 extends in the direction of the last outward turn of the dorsal 

 furrow to the frontal margin, a feature which is more emphasized 

 in the subgenus Thaleops. 



Another character more distinct in young stages is a median 

 depression on the cast of the cranidium, with a central ridge, both 

 extending from the occipital ring to the frontal border. The 

 integument shows only a corresponding faint median depression. 



