TRENTON CONGLOMERATE OF RYSEDORPH HILL 45 



marked off from the cheek® on each side of the glabella in the 

 young of Trinucleus c o n c e n t r i c u s ; and the lobation 

 of the glabella which in adult specimens of T. concent ricus- 

 has, become entirely obscured, but is faintly shown on the 

 young, is, in the specimens in the writer's hands, distinctly devel- 

 oped (see pi. 3, fig. 18). It is this feature on which Nicholson and 

 Etheridge would base the genus Tretaspis. 



The specimens from Kiysedorph hill, therefore, not only retain 

 features characteristic of the early stages of T. cone e n t r i c u e 

 to a mature or approximately mature size, but have certain of 

 these features., notably the lobation of the glabella and the orna- 

 mentation of the cheeks even more strongly developed than 

 those young stages. There can, therefore, be hardly any doubt 

 that this form represents a phylogenetic stage in the develop- 

 ment of the Trinucleidae, that is preceding Trinucleus proper 

 and partly repassed in the ontogeny of the latter. It seems to 

 the writer to be in full accord with our modern conception of a 

 genus to recognize this distinct stage by a separate generic term. 

 The features characteristic of this genus are also apparent in 

 Angelinas species, Trinucleus b u c c u 1 e n t u s , (tab. 41, 

 fig. 1) and T . f o v e o 1 a t u s , while his T . a f f i n i s shows 

 all these characteristics without the glabellar furrow, but 

 his suggestion, " an potius status juvenilis Trinuclei 

 seticomis?" would relegate this species into that group. 

 Our species agrees fully with T . (Treta s p i s) s e t i c o r - 

 n i s Hisinger, in the following features : the outline, sur- 

 face sculpture and lobation of the glabella, the details of 

 the marginal border with the exception that in T . r e t i c u - 

 i a t u s it is not bent vertically downward at the sides but only 

 at an angle of about 45°. No important difference appears in 

 the development of the thorax and pygidium. The principal 

 differences would, then, seem to lie in the crest on the glabella, 

 the less rounded and less convex profile and more trapezoidal 

 outline of the cheeks and their strongly reticulated surface. We 

 notice however that Angelin figures a form as T . a f f i n i s 



