LOWER SILURIAN. 



171 



The fossils thus far obtained from the rocks of the Acadian epoch 

 differ in species from those of the Potsdam. They include species of 



within the dorsal valve. Stricktaridinia of Billings may be the same genus, and, if so, 

 it antedates Rensselaeria. Cenlronella seems to be intermediate between Terebratula 

 and Waldheimia. Other genera, rarely met with, are Trigonosemus, Megerlia, Magas, 



Figs. 218-225. 



Fig. 218, Waldheimia flavescens ; 219, loop of Terebratula vitrea ; 220 id. Terebr'atulina caput- 

 serpentis ; 221, Spirifer striatus ; 222, same, interior of dorsal valve; 223, Athyris concentrica ; 

 224, 225, Atrypa reticularis, the latter dorsal valve. 



Argiqpe, appearing first in the Cretaceous, and Kraussia, Bouchardia, and Morrisia, 

 known only in recent seas, with a possible exception of the last. Stringoceplialus is 

 another genus, probably constituting a subfamily, occurring in the Devonian. 



2. Spirifer Family. — The genus Spirifer includes the common species, having usually 

 along hinge-line and distinct cardinal area (Figs. 221, 222). In A thyris (Fig. 223), 

 the hinge-line is much shorter, the hinge-area small or none, the beak contracted and 

 having a small round aperture. This genus is like Terebratula, in its narrow form and 

 beak without cardinal area, but has the spires of the Spirifers. Unelt.es has the beak 

 extravagantly prolonged, and a large opening beneath it. Cyrtia has nearly the same 

 extravagant prolongation of the beak, but with a large hinge-area, and a very small 

 opening left at the top of the pseudo-deltidium. Koninclrina is an imperfectly deter- 

 mined genus, resembling Productus in form, but differing internally. 



Among other genera and subgenera of this family may be mentioned Cyrtina, Retzia, 

 Merista, Nucleaspira, Trematospira, Rhynchospira , Charionella, etc. 



3. Rhynchonella Family. — The genus Rynchcmella (Figs. 226-228) contains plump- 

 ovoid or subrrigonal shells, usually narrower than high, and narrowing to the beak, 

 having usually a foramen and no hinge-area; generally a U-shaped flexure in the an- 

 terior margin of the shell. Pentamerus has a much fuller and more incurved beak, and 

 no area or deltidiuvn, though there is a triangular opening at the middle of the hinge, 

 which usually becomes closed in adult shells by the incurving of the beak. Ca.maro- 

 phoria is a rare genus of the Carboniferous and Permian. Porambcmiles, a very plump 

 shell of the Lower Silurian, near Rhynchonella'. Camerella of Billings is another genus 

 of this family, found in the Lower Silurian. Leptocoelia and Eatonia probably belong 

 to this family. Atrypa, Figs. 224, 225, which is referred to this family by Woodward, 

 on account of the arrangement of its spiral arms, narrows to the beak, where there is 

 no hinge-area or only a small one. 



