122 NACELLA. 



"The description given by Reeve is very good, but he does not 

 lay sufficient stress upon the beautiful raised concentric ridges. He 

 calls them striae, which term scarcely gives the idea of thread-like 

 lirations such as these. They are very closely packed and undulate 

 very prettily on and between the numerous radiating ribs. 



" The figure, except in outline and the position of the apex, gives 

 but a poor idea of this beautifully sculptured Patella. It represents 

 the number of ribs at about forty, whereas there are usually about 

 sixty. The specimens from Kerguelen's Island are a trifle narrower 

 and much more depressed than examples from the Falkland Islands ; 

 in fact, it is only near the apex that they are at all raised, and to- 

 ward the margin they are upturned, so that the dorsal surface is 

 concave, and this form of the shell certainly prevents the animal 

 from entirely concealing itself when adhering to a flat surface. But 

 this peculiarity of form only exists in adult specimens, for several 

 small ones are like ordinary species in this respect. The radiating 

 ribs are almost obsolete in the flat examples, but the undulating 

 concentric lirations, which are more prominent and farther apart 

 than in the type form of the species, define their position ; in young 

 shells they are more pronounced. Color generally uniformly 

 purplish slate, with the apical region ferrugineous ; interior similarly 

 tinted, but rather more deeply. One shell has a white border. 

 They are all very thin and fragile, and the edge is very liable to 

 break off in a line with the concentric raised lines of growth. 



" The animal has the sides and sole of the foot greenish-grey, the 

 edge of the mantle and gills pale buff, the tentacular filaments 

 on the margin of the mantle blackish except at their tips, tentacles 

 short and thick, pale buff, with a black spot above. 



" The frill-like expansion of the foot, similar to that of P. cenea 

 and P. kerguelensis, is a little above its edge, is bluntly serrated, and 

 interrupted beneath the head. 



" Teeth of the lingual ribbon slightly hooked, in pairs, scarcely 

 diverging; the central pair two-pronged, the inner prong much 

 larger, spear-head shaped ; the lateral pairs alternating with the 

 central ones are four-pronged, the innermost prong smallest, the 

 next two subequal, and the outside one situated nearly at right 

 angles to the rest of the tooth, about the same size or a trifle larger." 

 (Smith.') 

 N. CLYPEATER Lesson. PL 50, figs. 40, 41, 42, 43. 



Shell circular or rounded-oval, rather thin but solid, de- 

 pressed, the apex a little in front of the middle. Outer surface 



