998 PALEONTOLOGY OF NEW-YORK. 



This species is associated with S. varicosa ; but inaylw distinpiiished from that 

 one l)y the more munerous aiul smoother pliciitioiis ; a more depressed, smooth 

 aiid rounded mesial sinus ; the anterior inclination of tiie area, and the propor- 

 tionally greater length of the ventral valve, (which is a little greater than the dorsal 

 valve in that species,) and much greater than the height of the area. 



It also hears some resemblance to S. euruteines of Owen ; but in that one the 

 area is vertical and considerably less in height than the lengtli of tlie ventral valve, 

 and the latter is longer thmi the dorsal valve ; and although a smaller shell, the 

 S. segmenta has more numerous plications than 8. eurutetnes. Its nearest analogue 

 is the S. (uiffiista of the Ilanu'lton group. 



The figures 5, G, 7 and 8, Plate 31, illustrate the form and proportions of this 

 species : the cardinal extremities have been broken oft'. 



Geological foi-mation and localit)/. In limestone of the age of the Upper Hel- 

 derberg, at the Falls of the Ohio, and at Charleston landing, Indiana. 



8i)irifera arctise^iiienta. 



PLATE XXXI. 



Spirifer arctucgmentut : Hall, Tenth Report on the State Cabinet, p. 181. 1857. 

 " " Description of New Pal. Fossils, p. 91. 1857. 



Shell transversely semioval ; length less than one-third the width : 

 hinge-line equal to the greatest width of the shell, and terminating in 

 mucronate points. 

 Ventral valve the more convex, most prominent at the umbo, from 

 which it slopes regularly to the anterior and lateral margins : mesial 

 sinus angular, and distinctly defined quite to the apex of the shell ; 

 beak not incurved. Area flat, a little inclined forwards, striated longi- 

 tudinally ; fissure narrow and open to the apex. 

 Dorsal valve depressed convex, scarcely flattened towards the cardinal 

 extremities : the beak and central portion of the shell, together with 

 the linear area, slightly incurved. 



The surface of the ventral valve is marked by eight or nine angular 

 plications, which are slightly curved towards the front, and about three 

 of them only reaching the apex ; the remainder coalesce with an eleva- 

 ted ridge which borders the area. The plications on the dorsal valve are 

 pretty direct, the greater part of them terminating in the margin at a 



