222 STUDIES IN EVOLUTION 



As compared with Triarthrus, specimens of Trinucleus are 

 not very common at this locality, and although more than 

 fifty individuals of the latter have been obtained from the 

 collections presented to the Yale Museum by Professor Marsh, 

 not more than half a dozen of these are adult specimens, and 

 but three show any appendages. Young specimens of all ages 

 occur, from about 1 mm. across the cephalon upward, and in 

 all the eye-line and eye-tubercle are present until a width of 

 nearly 5 mm. is attained, when in the present species these 

 features dwindle and disappear, leaving no discoverable traces 

 in the adult. 



Two cephala of young individuals, without the free-cheeks, 

 are shown enlarged in figures 1 and 2 of Plate X. Figure 2 

 represents a specimen before the appearance of the perforate 

 border, and figure 1 gives a later stage, having two rows of 

 perforations around the head. On both specimens the eye- 

 line is clearly shown, extending somewhat obliquely back- 

 ward from the anterior lobe of the glabella to the central 

 area of the fixed-cheeks, enlarging slightly, and terminating 

 in a rounded node or tubercle (a, a, figure 2). 



In seeking for homologous features in other trilobites, the 

 genera Harpes and Harpides are immediately suggested, 

 since they have similar oculai ridges extending from the sides 

 of the glabella, and ending in a tubercle, which, in Harpes, 

 contains from one to three eye-spots, as determined by Bar- 

 rande. They further agree in having these visual organs on 

 the fixed-cheeks, while in all other trilobites with distinct eyes 

 the free-cheeks carry the visual areas. This type of eye 

 is thus quite different in its relations to the parts of the 

 cephalon, from that of Phacops or Asaphus, and more nearly 

 resembles the eyes of some of the Merostomata QBellinurus ), 

 as do also the triangular areas in the young Trinucleus, so 

 distinctly marked off from the fixed-cheeks on each side 

 of the glabella behind the eye-line. Adult Trinucleus and 

 Harpes have these areas much reduced and often obsoles- 

 cent. A spot or node in the median line on the glabella has 

 been noticed by many observers, and although its nature has 



