GENUS TREMATOSPIRA. 271 



or absence of a pseuclo-dcltidium covering the fissure I should not regard as of 

 specific importance, and the distortion of the heak of the ventral valve is a feature 

 common also to the C hamiltonensis. 



This species is probably the one described by Dr. White ; but he speaks of the 

 shell as small, while the one under examination is larjre for one of the ccnus. It 

 has also a greater ninnber 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 fonyiation and locality. In bedg of the age of the Hamilton group 

 near Iowa city, Iowa. 



GeXUS TREMATOSPIRA. 



Trematotpira : Hali. in Third volume of Pal. New- York, p 207. 



" " in Twelfth Report on the State Cabinet, p. 27. 18o9.» 



This genus was originally proposed by me to embrace a few forms, 

 having a general resemblance to Spikifera, 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 sufiicient to separate 

 them from SpiRiFf.RA 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 Vol. iii, Palaeontology of New- 

 York, in the years 1857 and 1858, were first published in the Twelfth Report on the State Cabinet 

 in 1859. 



