NAPLES FAUNA IN WESTERN NEW YORK, PART 2 



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regularly to the lower margin, more abruptly toward the crescent, and verti- 

 cally to the posterior hiatus. The surface markings consist of 12 to 15 

 primary ribs, not including the crescent ridge. These ribs are sharply 

 angular, and each bears a vertical, bladelike lamella extending along its 

 middle line. On each lateral slope of the primary ribs is a finer lamella, 

 and usually two more of similar size may be seen in the furrow intervening 

 between two ribs. Thus between the crested plications are four fine radial 

 lamellae, the two outer of which are the coarser and the other two some- 

 times obscured. The primary crested ribs are continuous and simple ; no 

 example has been observed showing increase in number from the early 

 parts of the shell outward. The rib bounding the anterior slope is shorter 

 than the rest, not reaching to the beak ; below it are a few of the finer 

 lamellae. The ridge bounding the crescent is broad and prominent. Its 

 summit is rounded and gives off a single series of spines, four to six in 

 number, the upper ones being short, the lower curved somewhat irregularly 

 outward and backward and the last, the longest, curved downward at the 

 tip. These spines are hollow and not inclosed on the under side, with the 

 exception of the terminal spine. They are formed by the upturning of 

 the contracted edges of the crescent and the body of the shell at periodic 

 cessations of growth. The surface of the crescent itself is marked with 

 low, rounded and broad plications alternating in size. These plications are 

 very oblique, passing from the region of the beak and the ridge backward 

 at a low angle and crenulating the margin, where, by the interlocking of 

 these crenulations on opposite valves, articulation is assisted. 



These shells are sometimes quite small, the most perfect of our speci- 

 mens not exceeding a width of 4 mm. Larger examples, however, attain a 

 length and width of 1 1 mm. 



Habitat. Genesee province ; Naples subprovince. Parrish gully, 

 Naples ; Briggs's gully, Honeoye lake ; Whetstone gully, Conesus lake ; 

 Havana glen, Schuyler co., etc. ; also in the lower shales at Attica, 

 Wyoming co., Wolf creek and the lower Portage falls of the Genesee river ; 

 rare in the Genesee shales, Canandaigua lake. 



