1896. | NEW YORK ACADEMY OF SCIENCES. 201 
the back. Though shorter and wider these objects are not 
unlike plates of Plumulites, not only do they have the same 
sharp, transverse ridges, but a similar arched band extends 
across the middle, from the apex. This genus when described 
was placed by its author among the Pteropods, but if I am 
right in having referred to it certain small fossils of the Para- 
doxides beds, there would seem to be features indicating affinities 
with the Crustaceans, and especially with the ancient Cirripedes, 
rather than the Pteropods. 
Stenotheca had thin calcareo chitinous shells similar to the 
tests of trilobites, sharply raised ridges parallel to the lower 
border ; the apical angle was bounded by two sides, one of which 
was convex and the other concave. The convex side is called 
dorsal, and the concave ventral, the third side is the aperture or 
base. 
STENOTHECA CONCENTRICA. (Plate XIV., fig. 1.) 
Stenotheca concentrica, Trans. Roy. Soc., Vol. III., pt. iv., p. 
59) pl. vi., fig. 11. 
Amended description. Outline of the compressed fossil rudely 
lenticular, the dorsal and ventral slopes being together equal to 
the strongly arched apertural margin. The apex varies from a 
bluntly rounded form to one which is prolonged into a short 
triangular point. For about one-third from the apex the sur- 
face has irregular undulations, about seven in number, concen- 
tric to the apex, and the rest of the test on the ventral slope 
about eight strongly marked ridges, concentric to the apex; 
these ridges are increased on the dorsal slope to the number of 
twelve or thirteen by the intercalation of additional ridges in 
the dorsal third. 
Sculpture. The surface appears to have minute pores and is 
ornamented with linear ridgelets which have a roughly parallel 
and sometimes anastomosing arrangement. The most typical 
form of ornamentation is a chevron or zigzag arrangement of 
minute ridges, visible only with a lens, and most marked on the 
outer part of the sheli; these chevron lines sometimes cross two 
of the concentric ridges before turning and usually run angling 
across the ridges. The chevron sculpture varies to an irregular 
wavy arrangement of ridgelets, and that to a more or less linear 
grouping, which is the usual appearance of the ornamentation 
at the apex: occasionally straight lines on the primary ridges, 
alternate with wavy or somewhat zigzag lines in the hollows 
between. Another occasional variety of the sculpture is that 
ofirregular wavy ridgelets crowned by little tubercles along the 
summit of the ridgelets; this gives the effect of a cancellated 
surface. 
