GASTROPTERON. 39 
the back above the gill; it is very small, white; oval, entirely open, 
without trace of a spire except the hook on the left margin. The 
lip is prolonged in a curved point above. It is entirely sulphur yel- 
len. Copulation always reciprocal. Length (of shell) 6, diam. 4 
mill. 
Port Dorey, New Guinea, on Zostera, at low water mark. 
Bulla lutea Q. & G., Voy. de l’Astrol. ii, p. 369, pl. 26, f. 40-44. 
Family GASTROPTERIDZ&. 
Shell wholly covered, consisting of a minute nautiloid, calcareous 
spire and a large open last whorl of very delicate membrane or 
cuticle. 
Body elongated, the fore part bearing a head shield, hind part 
nude, short, sack-shaped, the mantle edge conspicuous along the 
right side. Foot long, its borders produced in extremely wide lat- 
eral wings or pleuropodia. Stomach without plates; penis sack not 
grooved, and with a long prostate. 
Radula with the formula 5:1:0°1°5, the teeth as in Philine. 
This family is characterized by the enormous size of the lateral 
extensions of the fuot, which are used as swimming organs, instead 
of being folded over the back as they are in the preceding groups. 
The shell, moreover, is non-calcified, excessively thin and membra- 
nous except the minute spire which is white, calcareous and invo- 
lute. It will be remembered that the young of some other shield | 
headed Tectibranchs use the parapodia for swimming. 
Genus GASTROPTERON Kosse, 1813. 
Gastropteron J. F. J. Kosse, De Pteropodum ordine et novo ipsius 
Genera, p. 10 (1813).—VaysstERF, Rech. Zool. et Anat. sur les 
Moll. Opistobranch. du Golfe de Marseille, i, p. 39.—Brreu, Bull. 
Mus. Comp. Zool., xxv, p. 201.—FiscuEr, Journ. de Conchyl., 
1890, p. 349.—Gasteropteron of some authors.—Gasteroptera 
Buarnvy., 1825.—Parthenopia OKEN, Lehrbuch der Zoologie, 1815, 
i, p. 830.—Sarcopterus RAFINESQUE, Specchio delle Sci., ii, p. 11, 
(1814). 
Generic characters those of the family. Type G. rubrum. Gas- 
tropteron swims rapidly by means of its large parapodial lobes which 
are used as wings. 
Three species have been described: G. rubrum Raf. (meckelii of 
authors), of the Mediterranean and ocean coast of France, in which 
