GA8TROPTERON. 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. 

 Sulla lutea Q. & G., Voy. de PAstrol. ii, p. 369, pi. 26, f. 40-44. 



Family GASTROPTERID.E. 



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'O1'5, the teeth as in Pliiline. 



This family is characterized by the enormous size of the lateral 

 extensions of the foot, 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). VAYSSIERE, Rech. Zool. et Anat. sur les 

 Moll. Opistobranch. du Golfe de Marseille, i, p. 39. BERGH, Bull. 

 Mus. Comp. Zool., xxv, p. 201. FISCHER, Journ. de Conchy 1., 

 1890, p. 349. Gasteropteron of some authors. Gasteroptera 

 BLAINV., 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 



