Nelson on Tertiary Mollusca of Peru. 189 



greatest convexity the strong lines are supplemented by finer and 

 more numerous ones. 



Sutures very deeply impressed. No perfect specimens found except 

 young shells. A specimen with six whorls measures: length 76'90 

 millim. ; breadth 25 millim. ; breadth of upper whorl 12-6 millim. A 

 young specimen measures: length .30'1 millim. ; breadth 10*6 millim. 



This species seems almost as variable as abundant. Some of the 

 specimens resemble T. tigrina Kien., but may easily be distinguished 

 from that species by the greater convexity of the whorls, and 

 stronger revolving lines. On all mature specimens the finer striation 

 of the lower part of each whorl is very characteristic, but in younger 

 specimens the striations appear nearly iiniform from base to apex. 

 Some few specimens show occasional fine lines, intermediate between 

 the larger and stronger ones. The place of greatest convexity of the 

 whorls varies in a few specimens, owing to a flattening of the whorls. 

 Lines of growth very distinct on some specimens. 



Turritella bifastigata, sp. nov. 



Shell turreted, slender ; whorls twelve to sixteen, flat or slightly 

 concave, except the body whorl, which is regularly convex ; whorls 

 bordered on each side by a strong obtuse ridge. 



Intermediate spaces ornamented by fine raised, nearly equidistant, 

 revolving lines, about ten in the space of five millimeters. Sutures 

 small and narrow, or rendered indistinct by the development of the 

 bordering ridges. Body whorl somewhat convex, except in young 

 shells ; strongly wrinkled by the lines of growth, which, on well pre- 

 served specimens, are sharp and acute. Base of this whorl marked by 

 from seven to ten lines, nearly as strong as the ridges of the upper 

 whorls. Aperture rounded; outer lip thm anl slightly produced 

 below. A specimen consisting of the seven lower whorls gives the 

 following measurements: length 61 millim.; breadth 19" 1 millim.; 

 breadth of upper whorl 7 millim. Nine whorls from a younger 

 specimen gives: length 39'05 millim. ; breadth 10*6 millim.; breadth 

 of upper whorl 3*2 millim. 



This interesting species shows some resemblance both to T. plana 

 Nelson and T. goniostoma Val. But T. plana is a much stronger 

 shell, and lacks the bordering ridges, so characteristic of this species. 

 T. goniostoma Val. has only one bordering ridge, viz., on the lower 

 side of each whorl, while a central ridge gives to the whorl a slight 

 convexity, which this species lacks. 



