PECTUNCULUS. 167 



Habitat : Sandy and shelly gravel and nullipore on 

 every part of our coast from the Shetland to the Channel 

 Isles, in 7-90 fathoms. It is gregarious, as well as gene- 

 rally diffused ; and it occurs in all our upper tertiaries. 

 Sars has recorded it from the Loffoden Isles, and Lillje- 

 borg from Christiansund ; and it ranges southward on the 

 east to the iEgean, and on the west to Madeira and the 

 Canaries. Brocchi and Philippi have enumerated it as 

 fossil in the Subapennine and Sicilian tertiaries. 



This is the " Dog-cockle " of Da Costa. The animal 

 is sluggish and shy. I never saw it crawling. Mr. 

 Clark observes that " the animal does not execute a 

 direct progressive locomotion, but only turns the shell 

 round on its disk or from side to side." The great 

 thickness of its shell does not effectually protect this 

 succulent mollusk from all its enemies. I was in- 

 formed by Dr. Lukis that the dredge often brought up 

 large empty shells, with the valves united, but having 

 pieces broken off. This has been probably the work of 

 the cat-fish, whose enormously strong jaws and teeth 

 nothing but a solid stone can resist. Mr. Cleghorn 

 attributed the imperfect state of all boulder-clay shells 

 to this cause ; but with respect to Cyprina Islandica, 

 which is the most common shell in such deposits, I will 

 in the proper place suggest another explanation. Spe- 

 cimens of P. glycymeris vary considerably in the degree 

 of convexity, as well as in the obliquity of their outline, 

 and in colouring. The typical or usual form is more 

 produced at the posterior side, especially in the adult 

 state ; the variety pilosa is more orbicular ; in the va- 

 riety decussata the longitudinal strise are deeper, but 

 less numerous ; and small, round coloured spots distin- 

 guish the pretty variety nummaria. The largest speci- 

 mens I possess are about three inches in diameter. The 



