Mr. W. Clark on the Animals of the BuUidae. 107 



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 posterior divi- 

 sion of the head-disc from the neck, with the deep central sinu- 

 ations, 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, draws up the shell to 

 meet the extension, its edges catch and raise them to an erect 

 posture. 



The eyes, though not large, are distinct and fixed in those 

 species in which they are not obsolete, as in B. hydatis, and I 

 believe in all other Bullae at the anterior internal bases of the 

 so-called tentacular processes ; they are not raised, but immersed 

 in the surface of the skin : though they and the flaps or fins are 

 in rather an advanced position, they are only seen when the ani- 

 mal is on the march, from its habitude at other times of keeping 

 these organs under the shell ; but in those of thin texture they 

 may be seen through it. The foot, as in B. cylindracea, is one 

 entire lobe, divided as in the type at the under part from its 

 posterior linear accessory, which also lies within the mantle in 

 the narrow aperture of the shell, by a deep solution of con- 

 tinuity. In this minute creature it is difficult to observe the 

 connection and origin of the accessorial lobe with its principal ; 

 the deep groove or rather hiatus between the two portions of the 

 foot causes the anterior division to assume the appearance, as 

 M. Loven terms it, of ''■' solea brevissima /^ the mistake is very 

 natural ; the free floating margins resulting from its apparent 

 division from the posterior lobe, give the appearance of a com- 

 plete foot. The anterior part of the foot is rather shorter than 

 the head-lobe, ovately subquadrangular and rounded at the front 

 and posterior extremities, sinuated at the centre, grooved around 

 the margin, and not at all auricled. I have at the present date 

 observed thirteen of these animals for four or five days in sea- 

 water, but I never saw any reflexion of the mantle ; but the foot 

 at times is considerably reflected laterally on itself and ante- 

 rior part of the shell, and in quietude assumes the quadrilobated 

 character of the tribe. The march of this animal is much more 

 lively than in jB. cylindracea. The mantle forms an anal sinus 

 at the posterior part of the canal of the shell ; there is a deep 



8* 



