FAUNA OF THE TRENTON GROUP. 
25 
there are usually five or six. The major striae do not all extend to the 
beak, about every alternate one dying out at about the mid-length, 
and they are about 1 mm. apart at the front in large shells. Along 
the hinge margin of both valves are short, diagonal wrinkles, making 
an angle of about 45 degrees with the hinge. These wrinkles are 
narrow and close together, sometimes as many as fifteen on each side 
of the beak. The holotype is 14 mm. long and 21 mm. wide. The 
largest specimen is 16 mm. long and 24 mm. wide; the smallest seen 
so far is 10 mm. long. 
This species differs from the more common form identified as 
Plectambonites sericeus in it3 greater and more irregular convexity 
and in the uniform presence of wrinkles on the posterior margin. 
In its convexity it is somewhat like P. pisum Ruedemann 1 , but is a 
much wider shell. The presence of wrinkles on the margin, a feature 
known, but not very common, among Plectambonites of the Trenton, 
suggests the shell which Ruedemann described as Plectambonites 
sericeus asper James, but which is now known as Plectambonites 
rugosus (Meek). The species described by Ruedemann from Ryse- 
dorph is not, however, the same as the one described by James and 
Meek from the Eden of Cincinnati, and the Rysedorph form may 
appropriately he known as P . ruedemanni, taking the specimens 
figured by Ruedemaun 2 as the types. This is a much flatter shell 
than P. youngi, and its surface is characterized by exceedingly fine 
and sharp striae, the grooves between them being wider than the 
striations. Moreover there is almost no trace of difference in size; 
a suggestion of alternation is seen when glancing at some specimens, 
hut on close examination it is almost impossible to pick out the 
larger ones. 
Horizon and Locality. From the lowest Trenton (Lorette) beds 
at the power-house above Montmorency falls and at Lorette, Que. 
Types collected by E. J. Whittaker and the writer. Named after 
G-. A. Young, who has taken a very helpful interest in the problems 
of the Ordovician of Ontario and Quebec. 
Genus, Oxoplecia Wilson. 
Oxoplecia calhouni ~WHson s . 
Oxoplecia calhouni Wilson, Bull. Victoria Memorial Museum, 1, 
1913, p. 81, figs. 1-6, Pl. VIII, figs. 1-3. 
The status of this generic name is at present unsettled. It was 
proposed, with the new species 0 . calhouni as the type, to supply a 
1 Bull. New York State Mus., 49, 1902, p. 19, PI. 1, figs. 8-20. 
2 Bull. New York State Mus., 49, 1902, p. 18, PI. 1, flga. 6, 7. 
3 Prof. Foerste has recently examined the type-specimens of Oxoplecia 
calhouni and considers the genus distinct from Cliftonia. 
10711 — 4 
