BULLA. 283 



disc, mentioned in B. hydatis, with the groove that is the con- 

 tinuation of the vas deferens ; and such of the other organs 

 that could be observed in so minute a species scarcely differ 

 from the type. 



M. Loven mentions eyes imbedded in the skin ; no appli- 

 ances enabled us to detect them in this species ; but whether 

 they arc there or obsolete, that circumstance is only specific : 

 ex. gr. in Natica monilifera the eyes are obsolete, but in 

 N. Alderi they are easily seen ; the presence or absence of 

 these organs is therefore not of generic value. All the other 

 points being absolutely similar to the type, we are bound to 

 consider the genus Cylichna as not applicable to B. cylindracea. 

 We have a preparation to show that the linear posterior lobe 

 of the foot lies in the narrow aperture of the shell, agreeably 

 to the invariable characters of the Bullce. 



B. TRUNCATA, Montagu et Auct. et nobis. 



Cylichna truncata, Loven. 



C. truncata, Brit. Moll. iii. p. 510, pi. 114. B. f. 7, 8; (animal) pi. V.V. 

 f. 4. 



Animal subcylindrical, convolute, flake-white ; mantle hya- 

 line, quite plain, extending occasionally beyond the margins 

 of the shell. The head lobe is large, compressed, very slightly 

 auricled, sinuated in the centre, and edged with a light lead- 

 coloured line, which may be the reflexion through the hyaline 

 tissue of the leaflets within the anterior part of the sinus 

 between the lobes, mentioned as conspicuous in B. hydatis, 

 and presumed to be seen in the last species. 



The mouth is between the head lobe and that of the foot. 

 The animal carries the head rather beyond the foot on which 

 it lies, so that it may be mistaken for that organ. The pos- 

 terior division of the head-disc from the neck, with the deep 

 central sinuations, give rise to what I call the flaps or fins, 

 which in this animal are more extensive than in B. cylindracea, 

 and have the shape of short, flat, triangular pointed fillets, 

 lying like the ears of a hare close to each side of the neck ; 

 but when the animal in marching, after having fixed the foot, 



