28 



DR JOHNSTON S DESCRIPTIVE CATALOGUE OF THE 



Hah. Between tide marks in Berwick Bay, not common. 



Body oval, convex dorsally, of a uniform cream-yellow colour. Cloak 

 smooth, reticulated with minute clear spots, so as to appear almost 

 porous like a piece of fine sponge ; the margin thickish, plain, undu- 

 late, free, and sufl&ciently broad to conceal the foot when at rest. 

 Tentacula arising between the cloak and veil, superior, cylindrical, 

 short, formed of a membrane folded into a tube slit along one side. 

 Eyes small, black, one at the superior base of each tentaculum. 

 Veil above the mouth broad, somewhat triangular, produced at the 

 upper and outer angles, which are folded. Mouth shortly probosci- 

 diform. Space between the cloak and foot smooth, deep. Orifice 

 of the generative organs on the right side, placed very forward, tu- 

 bercle-like. Branchia arising immediately behind it, single, naked, 

 plume-like, and pectinated ; the posterior half free. Foot oval, taper- 

 ing posteriorly when in a state of extension, and projecting beyond 

 the cloak ; the margins undulate, plain. Shell concealed in the sub- 

 stance of the cloak, dorsal, ovate-oblong, depressed, with a minute spire 

 at one end ; brownish, thin, pellucid, strongly wrinkled concentrically, 

 and marked with a slight fossa from the apex to the opposite angle. 

 Length 6 lines, breadth 3^ lines. 



I am unable to describe the internal organisation of this interesting mol- 

 lusc ; but it may be permitted me to direct attention to the wonderful 

 structure of its oral organs, which I could not (although not unfami- 

 liar with analogous structures in congenerous species) view without a 

 feeling of indescribable pleasure and amazement : and, to the lovers 

 of the microscope, I am satisfied that few objects can afford a more 

 gratifying display. Within the soft parts of the mouth there lie two 

 thin oval plates, one on each side, reticulated in an inconceivably 

 minute and regular manner, after the fashion of the compound eyes 

 of many insects ; the meshes being diamond-shaped, and set with a 

 small obtuse process at each angle. Between these plates (which, I 

 presume, are a modification of the maxillae or jaws) the tongue is si- 

 tuated : a broad membrane, folded at the sides, and armed with innu- 

 merable little spines or teeth, arranged in close-set transverse series, 

 parting from a longitudinal medial line. The tongue is of a square 

 shape, rounded at the lower end ; to which is appended an inversely 

 heart-shaped piece of similar structure and appearance : the whole 

 fitted to rasp down the vegetable matter on which the animal feeds. 



The cloak is fleshy, but not fibrous ; and, in its composition, includes many 

 small crystalline spicula of carbonate of lime, which are also to be 

 found in the foot and branchia. These spicula are colourless, short, 

 cylindrical, and rounded at both ends ; and they seem to have no de- 

 terminate arrangement. I have found similar spicula, but larger and 

 more abundantly, in the tegumentary system of the Doris. 



Like the land slug, the Pleurobranchus progresses by obscure undulatory 

 motions of the foot ; but it justly claims the " bad pre-eminence" of 

 being superior in sluggishness and tardiness. 



The specific name of Blainville is unjustifiable ; and no authority can war- 

 rant our adoption of it. Montagu (the discoverer of the species) called 

 it Bulla plumula ;* and, although the generic name must be altered 

 to suit the progress of science, his specific name is sacred, and be- 

 yond the changeful caprice of any systematist. Blainville has done 

 some further wrong to Montagu, in ascribing even the discovery of the 

 mollusc to Donovan ; whose name, I may observe, is often substituted 



* He subsequently constituted with it a new genus, which he called Lamcllaria, a name 

 preoccupied in botany. 



