TATELLA. 431 



(so that when placed upon a level surface, the side mar- 

 gins alone touch it), of a yellow or ochraceous horn colour, 

 with the hlue rays often all but wholly obsolete, and almost 

 invariably of a lighter tint. In both states the interior 

 has a tendency to iridescence, and the exterior is nearly 

 smooth, having merely some obsoletely raised but numer- 

 ous radiating stria?, which for the most part are more 

 conspicuous behind than in front, and some most minute 

 and densely set concentric striulse. In the more solid form, 

 however, the internal opalescence is much more beautiful, 

 and often exhibits a brilliant lilac or violet iridescence ; 

 and the external sculpture, besides being more pronounced, 

 has the stages of increase not unfrequently strongly indi- 

 cated. The margin is entire ; the vertex obtuse, and much 

 reclined. 



The younger shells of the typical and undistorted form 

 are much depressed ; the adult are conical convex ; the 

 more elevated the specimens the less marginal does the 

 vertex become. In latvis the dusky subinternal spot, 

 which almost always lies beneath the vertex of the shell, 

 is usually smaller, and is often nearly wholly absent. In 

 the typical pellucida a short central linear ray of the same 

 hue not unfrequently precedes this stain. We possess 

 specimens nearly an inch long, but the average size of 

 examples does not exceed two-thirds of that measurement. 



The animals of the two varieties are exactly alike in all 

 essential characters, varying only in colour. We find that 

 of lavis usually white or yellowish ; that of a well-grown 

 pellucida, as taken in the Hebrides, usually orange. The 

 comparatively greater exposure to light of the latter form 

 may account for this difference. Mr. Clark has examined 

 a very large series of the animal in all stages of growth, 

 from the pellucida, only a one-twentieth to a quarter of an 



