222 



coast (Godwin- Austen) , Clyde beds (Smith), Mamma- 

 lian and Red Crag (Wood) . The account of its geolo- 

 gical distribution is not satisfactory. N. catena does 

 not appear to inhabit any part of the arctic seas, like 

 other Mollusca whose remains are found in formations 

 for that reason assigned to the glacial period. More- 

 over the coloured markings of this species are not exhi- 

 bited in the Crag shells so named by Mr. S. Wood, 

 although they are retained in his N. millepunctata. Its 

 foreign range extends from Bohuslan (Loven) to Corsica 

 (Requien) . 



It was first described and figured by Lister, and is 

 the " English chain-headed sea Button-shell " of Petiver. 

 Specimens in Macgillivray's collection from Aberdeen 

 measure full two inches in length and breadth. I 

 dredged at Guernsey a small one the spire of which is 

 reversed or sinistrorsal. The fry are globular, orange- 

 colour, and umbilicate ; they assume the purplish-brown 

 markings after they are excluded from the leathery 

 band noticed in my account of the genus, and which in 

 the present case when dry looks not unlike a piece of 

 thin Scotch oaten bread. Bouchard-Chantereaux informs 

 us that the sexual coition lasts many hours ; and that 

 the spawn-envelope consists of a great number of rounded 

 cells, each containing from 12 to 15 fry, which emerge 

 in succession at an interval of two or three days after 

 at least two months of foetal life. The eggs are laid 

 usually in March and April, and the young are produced 

 in May and June. This mollusk was justly admired by 

 Mr. Clark, who says, " When just taken, in vigour, and 

 immersed in sea-water, it is scarcely possible to con- 

 template a more beautiful and interesting object, with 

 its shell rising as a globular pyramid from its immense 

 circular disk, elegantly marked with fine dark lines on 



