IIS NUCUUDJS. 



covered by a thin greenish oil-coloured epidermis, everywhere 

 (near the slopes excepted) conspicuously decussated by irregularly 

 concentric coslellar or lamellar wrinkles, pairs of which often be- 

 come confluent anteriorly, and close-set radiating raised lines. 

 Anterior extremity somewhat tapering, chiefly projecting below 

 the middle, rounded ; posterior end rotundately subangulated 

 below, its dorsal slope very decided. Ventral margin closely cre- 

 nated within, more rising in front, at times much arcuated in the 

 middle, at times (in the very inequilateral specimens) only slightly 

 so. Lunular area rather large, pouting, more or less angularly 

 defined, for the most part subgranosely wrinkled near the cir- 

 cumference, smoothish in the middle : opposite dorsal area rather 

 flattened, usually obscurely barred with wavy pliciform wrinkles, 

 which generally become obsolete with age. Teeth about twelve 

 on the shorter side, and twice as many on the longer. — There 

 are three known variations. The North British form, where 

 the wrinkles are almost lamellar, and the shape is somewhat more 

 ovate; the N. African form, where the wrinkles are costellar 

 and shelving, and the shape triangular ; and the Mediterranean 

 form, where the sculpture is comparatively undeveloped and the 

 form intermediate between the two previously indicated. Mr. 

 Cuming possesses an aged example (seven-eighths of an inch iu 

 length) in which the radiating lines appear incised near the lower 

 margin, and the smooth lunular area is devoid of the ordinary 

 sculpture, projection, or definement. 



2. N. nucleus, Lin., f. 121,122,159. — T. obovato-trigona, raro 

 obliqua, satis ventricosa, valida, cute lutea vel olivacea et nun- 

 quam nitida induta, nunquam colore radiata, in medio sublsevigata, 

 utrinque tenuissime concentrice subplicato-corrugata et (postice 

 prsesertim) minutissime radiatim lirata. Extremitas brevior ro- 

 tundato-obtusangularis, inferne eminentior. Margo ventralis satis 

 argute crenulatus, arcuatus, postice acclivis. Nates promiuentes, 

 inelinate. Lunula nitida, concentrice corrugata, neque granis 

 nee lineis radiantibus sculpta. Area aut laevigata aut concentrice 

 sculpta.- — Obovate-trigonal, almost always longer than broad, sel- 

 dom oblique, strong, moderately ventricose, clothed with an 

 epidermis that is never polished or colour-rayed : general surface 

 smoothish in the middle but very finely concentrically and sub- 

 pliciformly corrugated at both ends, and, under a lens, (particu- 

 larly behind) perceptibly, though faintly, closely radiatingly lyrated. 

 Inequality of the sides decided, yet not remarkable ; extremity of 

 the anterior side, which is produced and tapering, rounded off; its 

 chief projection basal. Posterior end rotundately obtusangular, 



