GENUS TREMATOSPIRA. 
271 
or absence of a pseudo-clelticlium coveifiiig Hie fissure I should not regard as of 
specific importance, and the distortion of the beak of the ventral valve is a feature 
common also to the C. hamiltohensis. 
This species is probably the one described by Dr. White ; but he speaks of the 
shell as small, while the one under examination is large for one of the genus. It 
has also a greater number of plications, both on the fold and sinus as well as on 
the sides of the shell. These differences may be due to age and condition. 
The figures 53-55, Plate 44, illustrate the characters of this species. The sinus is unequal 
in depth, but its limits are not quite sufficiently defined in the figure, and I have added a 
dotted line (. . . .s) beneath, indicating the limit of the sinus on the right side. 
Geological formation and locality. In beds of the age of the Hamilton group 
near Iowa city, Iowa. 
Gem s Trematospira. 
Trematospira : Hall, in Third volume of Pal. New-York, p. 207. 
“ “ in Twelfth Report on the State Cabinet, p. 27. 1859.* 
This genus was originally proposed by me to embrace a few forms, 
having a general resemblance to Spirifera, but without the extended 
hinge-line and area; or with the latter feature uncertain or undefined, 
and wanting in the characters of a true area. Such at least is the cha¬ 
racter of the specimens originally examined; some of which approach 
in extended form to Rhynchonella. 
Up to this time, comparatively few species are known; and of the 
greater part of these, few individuals have been obtained, so that our 
knowledge of the interior structure is still imperfect. In all the species 
possessing the peculiarities of external form noticed, we find a punctate 
structure of the shell; a character which alone is sufficient to separate 
them from Spirifera proper, or from Rhynchonella. 
The genus was founded originally upon species from the Lower Hel- 
derberg group of New-York, including a single species from the Niagara 
group. I have, since that time, observed similar external features in 
specimens from the Lower Silurian rocks of Ohio and elsewhere; while 
a single species from the Hamilton group, first described by me as Atrypa 
*The description of this and other genera of Brachiopoda, printed in Yol. iii, Palaeontology of New- 
York, in the years 1857 and 1858, were first published in the Twelfth Report on the State Cabinet 
in-1859. 
