OF NORTHUMBERLAND AND DURHAM. Ill 



inncli diminished that it leaves little more than a spot, and in 

 that case the species might be taken for the last ; but it may 

 always be known from it by the thinness of the general envelope 

 and the more variegated reddish colour of the inclosed animals. 



3. B. viRESCENS, Aid. and Hanc. 



General envelope moderately thick, olive-brown coloured, 

 sprinkled with yellow spots. Individuals forming circles of from 

 six to twelve. Colour grass-green, varying to greenish yellow, 

 and occasionally to pale grey ; the colour is in most cases con- 

 fined to the lower part of the animal, the upper portion being so 

 nearly the colour of the envelope as to be with difficulty distin- 

 guished from it. Branchial apertures large, with a faint red 

 margin ; the tentacular filaments very conspicuous within them, 

 of a pale yellowish colour. When magnified, the colour of the 

 body is shown to be formed of a copious sprinkling of opake 

 spots, mostly confined, as before stated, to the lower part of the 

 animal, but sometimes extending round the disc. Common cen- 

 tral apertures largish, margined with red. 



Not uncommon on the under side of stones within tide-marks, 

 along with the last. The arrangement of the markings distin- 

 guishes it from the B. smaragdus of Milne Edwards. 



4. B. POLXCYCLUS, Sav. 1 



General envelope thin, dull greenish brown. Individuals 

 forming circles of from 4 to 9 each, generally of a purplish hue, 

 occasioned by a spotting of claret-colour and blue intermixed 

 with spots of opake white ; the white usually forming one or two 

 large blotches in the centre of the body on a dark blue ground 

 and there is also a circle of white, or sometimes reddish, spots 

 round the disc. General apertures rather small; the edges 

 thickly sprinkled with white and blue. 



Boti-yUus jwlycycliis, Forb. and Hani. Brit. Moll. i. 21. 



On the under side of stones within tide-marks, not rare, but 

 less frequent than the three preceding. 



5. B. CASTANEUS, Aid. and llanc. 



General envelope very thin and pellucid, with a few scattered 

 brown marks and black punctures on the margin. Individuals 

 large, placed in irregular circles of from 6 to 8, of a chesnut 



