NO. 2124. THE WEST AMERICAN SPECIES OF NVCELLA—DALL. 559 



In the other northwest coast species the lamellations do not 

 become marked ; in extreme cases they convey the impression of low 

 close imbrication, or they may be reduced to mere elevated lines or 

 even be altogether absent. 



In only one of the species, JV. emarginata, do we find nodulation of 

 the spirals, and this not accompanied by any axial ribbing. 



The suture is simple and closely appressed in this group, though 

 the proximity of an elevated spiral, as in N. canaliculata gives the 

 shell an aspect of being channeled at the suture. 



The whorls are usually of a moderate convexity modified in par- 

 ticular mstances by the external sculpture. There is little variation 

 in their numbers within the species; it frequently happens that the 

 short stumpy specimens and the slender elevated ones prove on 

 examination to have the same number of post-nuclear whorls. The 

 variation between specimens fully adult and of the same general 

 type rarely exceeds one whorl, and that toward the apex of the shell. 

 In coiling there is great variation more or less correlated with 

 environment. 



The sheltered rocky beaches of a weU-protected harbor will afford 

 slender elongated and lamellose specimens with small apertures. 

 The outer rocks exposed to the ocean surf have short-spired, rela- 

 tively smooth, wide-mouthed shells, which afford the least leverage 

 to the waves. For, washed from his perch and carried to the muddy 

 bottom off the shore by the undertow, an adult Nucella can hardly 

 survive ; and those offering the least friction and having the stronger 

 hold on their situs are most likely to survive. There is also a con- 

 nection between the situs and the shell which is less easily explained, 

 and that is that, on rough surfaces such as an "oyster reef," or bar, 

 the specimens of lamellosa are almost unanimously rough and lami- 

 nate, while in undisturbed water on rocks with sandy surroundings 

 the finest and most delicate development of lamellae and crenulations 

 is to be found, according to the reports of collectors. In aU cases 

 Nucella seems to prefer a rocky habitat, especially if it affords yoimg 

 oysters or other sessile or sluggish species serving it as food. 



The siphonal fasciole in these shells is usually strong, with the caUus 

 of the pillar lip folded over the inner half of its cavity. The elevation 

 or depression of the shell seems to have no effect on the axis of the 

 pillar, but occasional specimens, usually old and more or less patho- 

 logic, show a decided umbUical chink. The canal in well-developed 

 specimens is rather strongly recurved. 



The aperture in all the species, when mature, is margined by a 

 continuous callus, sometimes thin and closely appressed, but in solid 

 and heavy specimens thick, with an elevated edge on the body and 

 pUlar. The outer lip, while hardly reflected, has a certain flare and 

 is shghtly expanded. The piUar lip is smooth and in exceptionally 



