DESCRIPTION OF SPECIES GASTROPODA 621 



little convex and gently converging in front of the eyes, the anterior 

 margin broadly rounded; glabellar furrows and lobes not well preserved 

 in the type specimen, but a large, strongly convex basal lobe is present, 

 and apparently two lateral furrows on each side which are in close prox- 

 imity to each other and to the furrow bounding the basal lobe; cheeks 

 sloping steeply from the eyes to the marginal furrow, where they are de- 

 flected nearly horizontally into the marginal border; eyes not preserved 

 in the specimen. Thorax with ten segments, the axis more than one- 

 third the width of the body, well defined and strongly convex; proximal 

 portion of the pleura nearly horizontal, the distal half bent abruptly 

 downward at an angle of about 120 degrees. Pygidium rather short, 

 subsemicircular in outline, but not well preserved in the type specimen. 



The dimensions of the type specimen are: Total length, 55 milli- 

 meters; length of cephalon, approximately 19 millimeters; width of 

 cephalon, 35 millimeters; convexity of cephalon, 13.5 millimeters; con- 

 vexity of thorax, 12 millimeters ; length of pygidium, 9 millimeters. 



Remarks. — The type specimen of this species is a somewhat weathered, 

 complete individual, upon which some of the characters are obscure. A 

 few imperfect pygidia have been observed which probably belong to the 

 same species; also an occasional broken genal spine. The species is per- 

 haps more nearly like P. missouriensis than any other, but that species 

 is proportionately longer, with a glabella which is somewhat broader and 

 subtruncate in front. The costse parallel with the margin in the mar- 

 ginal border of the head seem to be a characteristic feature of the species. 



Correlation 

 in general 



In the correlation of the Fern Glen fauna it is necessary to compare 

 it with the faunas of the Chouteau and Burlington limestones, with that 

 of the Saint Joe marble of Arkansas, the New Providence shale of Indi- 

 ana and Kentucky, and the Lake Valley beds of New Mexico. 



RELATION OF THE FERN GLEN TO THE CHOUTEAU AND BURLINGTON 



The relations of the Chouteau limestone as a formation have been 

 much misunderstood. The almost universal custom of considering it as 

 the upper member of a threefold classification of all of the Kinderhook 

 beds of the Mississippi Valley region is due to an entirely mistaken in- 

 terpretation. In the region of its typical development in central Mis- 

 souri, the Chouteau represents the entire Kinderhook interval and is 

 probably contemporaneous in part with the Louisiana limestone of north- 



