TRENTON CONGLOMERATE OF RYSEDORPH HILL 43 



«pecimeii shows seven, the large nine annulations; only faint 

 indications of pleural grooves on the nearly smooth, almost flat 

 sides; margin beveled, concentrically striated. 



Dimensions. Width of cranidium 13 mm, length 6 mm, greatest 

 width of glabella 3.4 mm. 



Horizon and locality. Nearly all specimens were obtained from 

 two small pebbles of black compact limestone, where they were 

 associated with Trenton fossils. (Group 5) 



Observations. This species differs too obviously from the com- 

 mon Trinucleusconcentricus and the similar T r i n u- 

 <3leus bellulus to necessitate a long exposition of these 

 differences; as most striking may be mentioned, the glabellar 

 fossae, the tubercles, the eye line, the structure and profile of 

 the marginal border, and the pitted reticulate surface of the 

 cheeks and glabella. While this species differs so markedly from 

 other American species, it shows the closest similarity to a group 

 well known in Europe, the typical form of which is T. seti- 

 c o r n i s Hisinger, which in the fuller discussion given to this 

 form and its characters by Nicholson and Etheridge^ is shown to 

 exhibit at the best only varietal differences from T. bucklandi 

 Barrande. By Boemer and Freeh the latter species is united with 

 T. seticornis. This species is specially characterized by the 

 presence of the glabellar pits, the apical tubercle on the glabella 

 and the ocular tubercles, the eye lines, the deep dorsal furrow 

 and the structure and profile of the marginal border. 



McCoy repeatedly (1849, 1851) propoised the subdivision of the 

 genus Trinucleus into two subdivisions — Trinucleus proper and 

 Tretaspis McCoy, the latter characterized by " the presence 

 of five thoracic segments, the furrov^'s on each side of the 

 base of the glabella, a diagonal line crossing the cheeks, 

 approximately occupying the position of the facial suture and 

 usually a small apical or culminating tubercle ocular (?) in char- 

 acter.'^ This subdivision was adopted by Salter in 1857, who 

 thus defines Tretaspis McCoy; "Ocular tubercle distinct; eye 



1 Monograph Sil. foss. Glrvan district in Ayrshire. 1830. Fasciculus 2, p. 190. 



