143 



ON THE ANATOMY OF FTEROCERA, WITH SOME NOTES ON 

 THE CRYSTALLINE STYLE. 



By M. F. WooDWAED, Demonstrator of Zoology, lioyal College of 

 Science, London. 



Bead May Wth, 1894. 



PLATE XI. 



It might at first sight seem unnecessary to redescribe the anatomy 

 of Pterocera, figures of which appear in all our leading text books ; 

 these, however, are all derived from one source,' and that original 

 is itself inaccurate in some important respects. 



The specimen on which the present observations were made was 

 kindly given me by Prof. Haddon, who collected it in Torres Straits ; 

 it had been removed from the shell and carefully preserved in spirit ; 

 the absence of the shell, however, prevents me from determining the 

 species. 



Pterocera is a Prosobranchiate Gastropod belonging to the Taenio- 

 glossate division of the Pectinibranchiata and to the family Strom- 

 bidse. It closely resembles the allied genus Stromhus, both in its 

 shell and the peculiar form of the head, foot, and operculum. 



Very good figures of the living animal are given by Quoy and 

 Gaimard,' and except for the fact that my specimen was a spirit one, 

 and, consequently, the foot and head were somewhat contracted, 

 there were no differences in the external characters. The enormous 

 size of the optic stalks and the eyes is very noticeable in the 

 Strombidae, and their position in relation to the peristome is very 

 marked ; the left optic stalk occupies the anterior canal with the 

 siphon, while the right one is lodged in that notch in the peristome 

 so characteristic of this family. 



The foot in these forms is long and narrow, and the creeping disc 

 seems to be entirely wanting ; it is divided into two well-marked 

 portions — an anterior heel-shaped structure, rather contracted in 

 my specimen, and a posterior pointed toe, bearing on its dorsal 

 surface the long, pointed, nail -like operculum, which projects back 

 freely from the foot. 



These snails have entirely lost the creeping method of progres- 

 sion, and are said to get over the ground by a series of leaping 

 movements. In the allied genus AporrJiais the foot is not quite so 

 narrow, but even here the creeping habit has been lost, the animal 

 progressing by digging its shai-p operculum into the ground and then 

 extending its foot, thus forcing the body and shell forwards ; the foot 



^ Voyage de I'Astrolabe, Zoologie par MM. Quoy and Gaimard, torn. iii. p. 878, 

 pi. xlix. fig. 18. " Strombus lambis.^' 



