PLEUROBRANCHUS. 
Genus PLEUROBRANCHUS, Cuvier. 
Testa marina, quam corpus suum multd minor, subtiis 
pallium celata, membranacea, intits tenuissimé cal- 
carea, spiralis, aperta, expansa, apice spirali. 
Shell marine, much larger than its body concealed 
beneath the mantle, membranaceous, very thinly 
calcareous within, spiral, open, expanded, with 
spiral apex. 
The testaceous membranes of Pleurobranchi differ 
from those of Aplysie chiefly in the character of the 
apex, which in the former shows a distinct, although 
flattened spiral coil, and in the latter is produced, in- 
curved, and covered by a reflected edge which is more 
or less callous. The most characteristic species are 
Pleurobranchus membranaceus and P. plumula, which 
are both British. There is one species described by 
D’Orbigny in Central America, and several from the 
Mediterranean. No doubt when these animals and 
their shells are better known they will be found to 
possess as large a geographical range as their near 
allies the Aplysiz. 

Species 1. (Fig. a, b, Mus. Jeffreys.) 
PLEUROBRANCHUS PLUMULA. Ple. testd oblongd, angus- 
tata, fulvd, levigatd, concentrice undulata ; apice 
subterminali, compresso ; margine superiori, supra 
apicem pauld elevato, utrinque subrotundo ; labio 
externo rectiusculo, medio vix contracto; margine 
dorsali 
pauld producto, rotundato. 
THE LITTLE FEATHER PLEvRoBRANCHUS. Shell oblong, 
narrow, fawn coloured, smooth, concentrically 
vix convexo ; margine terminali anticé 
undulated ; apex subterminal, compressed, slightly | 
rounded; outer lip rather straight, very slightly 
contracted in the middle ; dorsal margin very little 
convex; terminal margin a little produced in 
front, rounded. 
Montagu. Testacea Britannica, p. 214, pl. 15, f. 9. 
Hab. Britain. 

Species 2. 
(Copied.) 
PLevroprancuus Peroni. Ple. testéd parva, subovatd, 
concentricé rugatd; spird valdé incurva ; dorso 
rotundo, 
Péron’s Prevroprancuus. Shell small, subovate, con- 
centrically wrinkled; spine much incurved, back 
rounded. 
Cuvier. Ann. du Mus,, v. 5, p. 269, pl. 18, f. 1. 
Hab. Sicily, 

(Copied.) 
PLEUROBRANCHUS AURANTIACUS. Pe. 
oblonga, solidd, auriculiformt, succined, quadrata ; 
Species 3. 
testa minima, 
apice anticé terminal. 
THE ORANGE PLeuROoBRANCHUS. Shell very small, 
oblong, solid, auriculiform, amber coloured, quad- 
rate ; apex terminal in front. 
Risso. Journ. de Physique, p. 87, p. 374. 
Hab, Sicily. 

Species 4. (Fig. a, 6, Mus. Sowerby.) 
PLEUROBRANCHUS MEMBRANACEUS. Ple. testdé oblongo- 
subquadratd, compressissimad, fulvd, iridescentt, 
intis tenuissime testaced; pallidé rosed; concentrice 
undatim rugata ; apice valdé incurvo, compresso ; 
margine superiori rectitisculo, supra spiram subre- 
flexo ; labio externo convexo, infra magis expanso ; 
margine inferiort lata, utrinque subquadratda ; 
margine dorsali convexo, tenuiter reflexo. 
Tus MEMBRANACEOUS PLeuRoBRaNncuuS. Shell oblong 
subquadrate, much compressed, fawn coloured, 
iridescent, within very thirly testaceous, pale rose, 
concentrically wrinkled in waves; apex much in- 
curved, compressed; upper margin rather straight, 
alittle reflected above the spine, outer lip convex, 
more expanded below; lower margin wide, sub- 
quadrate at both ends; dorsal margin convex, 
thinly reflected. 
Montacu. Lamellaria membranacea. Trans. Linn. Soc. 
Hab. Britain. 
By far the largest and most beautiful species known. 
August, 1869. 
