BULLA. 275 



the shell ; that part of it covering the branchiae, heart, bladder, 

 matrix and testis is of firm texture ; the portion within the 

 posterior part of the spire, containing the liver, ovarium and 

 oviduct, is a mere film, but stronger than would be supposed 

 from its tenuity ; at the posterior end of the shell the dorsal 

 range is completed by the reflexion on it of the hindermost 

 part of the pedal lobe. The anterior part of the under lobe 

 forming the foot is precisely of the same length and width as 

 the upper one, but more posteriorly it spreads like a fin, 

 which is reflected on the sides of the shell; the disk then 

 pursues its course for two-thirds of its length posteriorly, 

 at which point its continuity is broken by a deep fissure, 

 which divides the plate, without injuring the fabric; on 

 the right side near this channel the anus debouches; the 

 pedal plate is then continued to the posterior end, becoming 

 wider, and there, as before stated, it is reflected on the 

 posterior part of the shell. 



It appears then that there are but two lobes, the upper or 

 tentacular, and the pedal one, which though interrupted by the 

 deep groove, is in reality a single plate, but from its posterior 

 and anterior lateral reflexions, together with the tentacular 

 disk, gives the animal the characteristic quadrilobated ap- 

 pearance of the tribe. The object of the pedal fissure is 

 probably to give flexibility to the foot, and allow the lobes to 

 act as fins, as the animal is equally an adept in natation and 

 reptation. 



Twenty years ago I observed hundreds of these creatures 

 swimming and creeping on the fine mud in the lakes of the 

 Mount Pleasant Warren near Exmouth ; they however sud- 

 denly disappeared from the locality, and not one has been 

 seen for many years ; the animals now described were obtained 

 near Swanage, Dorset. 



The large circumferential canal of the separation of the 

 upper and lower lobes is much more decided on the right than 

 on the left side, as from the former we have a view of the 

 orifice of the verge, the open seminal duct, and the common 

 cavity of generation, the testis, and points of the branchiae. 

 The eyes are very distinct, situate far back on the tentacular 



T2 



