CryptoHthus tesselatus Green 



(See pi. 35) 



i Lateral view of exfoliated glabella showing interior cast of 

 median eye tubercle, x 5. 



2 Lateral view of interior cast of eye tubercle, showing contrast 



between smooth upper and pitted lower surface of cast of 

 tubercle, x 30. 



3 Top view of same. Apex of cast of tubercle slightly rubbed or 



fractured (tubercle indicated by arrow), x 30. 

 Trenton limestone. Near Plattsburg, N. Y. 



Trinucleus bucklandi Barrande 



4 Specimen with exfoliated cephalon, exhibiting the lateral and 



median eye tubercles. (From Barrande). 



5 Exfoliated cephalon, showing lateral eyes and median eye 



tubercle with pit (indicated by arrow), x 5. 

 Ordovician. Girvan, Scotland. 



Isotelus gigas Dekay 



6 Cephalon of adolescent specimen with distinct median eye 



tubercle (indicated by arrow), x 2. 

 Trenton limestone. Trenton Falls. 



Asaphus expansus (Linn.) 



7 Interior cast of median eye tubercle, x 5. On the left a por- 



tion of the very thick test is shown. 

 Ordovician : River Wolchow, Gouvern. Petersburg, Russia. 



Illaenus ioxus Hall 



8 Cranidium, showing small median eye tubercle. Natural size. 

 Silurian: Rochester shale. Lockport, N. Y. 



Cheirurus niagarensis (Hall) 



9 Partly exfoliated cranidium, showing pit in interior cast in place 



of median eye tubercle (marked by arrow). Natural size. 

 Silurian : Rochester shale. Rochester, N. Y. 

 All originals, with the exception of that of figure 4, are in the 



New York State Museum. 



2 16 



