GASTEROPODA. 127 



PORCELLIA NAIS. 



PLATE XVI. FIGS. 27,28; and PLATE XXVI, FIGS. 26-28. 

 Oyroceras Nais, Hall. Descriptions of New Species of Fossils, etc., p. 40. 1861. 

 Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 68. 1862. 

 Porcellia " " lb., Explanations pi. 6, figs. 5 and 6. 



Porcellia " " Illustrations of Devonian Fossils : Gasteropoda, pi. 16. 1876. 



Shell consisting of about four volutions, which are somewhat rapidly expanded 

 from the apex towards the aperture ; volutions subangular and nodose on 

 the sides, and rounded on the back ; the dorsum a little elevated on each 

 side of the mesial band; the ventral side sloping abruptly into the'umbil- 

 ical depression. The aperture has probably been somewhat broadly sub- 

 reniform. 



Surface of the umbilical slopes of the volution marked by strong, rounded, 

 transverse striae, which are sometimes bifurcated toward the side of the 

 shell, and are crossed by revolving lines, giving it an undulated or sub- 

 nodose aspect. The back of the shell, in a partial , cast, is conspicuously 

 marked by flattened revolving bands, which are crossed by less strong 

 transverse striae. The dorsum is marked by a narrow depressed band, on 

 which the striae make an abrupt retral bend, indicating the sinuosity in 

 the margin of the aperture. About half-way from the dorsal line to the 

 angular side of the volution, commence low elevations which become strong 

 oblique ridges or elongate nodes, limited by the angular margin, and not 

 passing to the ventral region nor marking the umbilical slope of the 

 volutions. 



The larger specimen, which is figured on plate 16, preserves about one 

 volution and a half; it is imperfect towards the aperture, and the inner volu- 

 tions are broken away. It has a diameter of about two inches. 



A fragment, consisting of nearly three of the inner volutions, while preserv- 

 ing the general features already noticed, shows the dorsal side and umbilical 

 slopes of the volutions to be cancellated by distinct elevated equal striae, which 

 are conspicuously nodulose at their crossing ; it also shows that the entire shell 

 consists of at least four volutions. 



