1)1 



Figure. 



Rostellaria fusus. Plate 7. Fig. 36. Shell, showing its graceful 

 fusiform growth, and digitated lip. — From ike collection of Br. Knapp, 

 of Edinburgh. 



Genus 5. PTEROCERA, Lamarck. 



Animal ; similar to that of Rostellaria. 



Shell ; ovately oblong and ventricose, the last whorl being conside- 

 rably inflated, and ending in an elongated canal of a structure 

 similar to the digitations of the lip ; columella and aperture 

 peculiarly ridged ; outer lip developed at maturity into a con- 

 spicuously clawed or digitated wing, and sinuated towards the 

 lower part. 



The soft parts of Rostellaria, Pterocera, and Sirombus, are so exactly 

 similar in character, that M. Deshayes almost doubts the propriety of their 

 being divided into separate genera, and more especially as they are distin- 

 guished in a notable manner from the proximate kinds by a modification of 

 structure, of which there is no analogy in any other genus of the class. The 

 disc is divided in a manner which obliges the animal to leap*, as it were, 

 instead of to crawl, and the tentacles are curiously two-forked, the stouter 

 branch, a modification of that portion which is commonly pedunculated, 

 being destined for the support of an eye of unusually large proportions. 



The eye of these genera appears to be more highly developed than in 

 any other of the Gastropods ; it is described as covering the summit of this 

 stout, truncated, tentacular branch, and composed of a transparent homy 

 material containing an iris, differing in colour according to species, for the 

 transmission of rays of light into an inner chamber ; an organism plainly 

 adapted for seeing. 



The distinguishing peculiarity of this family, as regards the shell, the 

 wing-like expansion of the mature lip, presents itself under such different 

 phases in the genera just spoken of in reference to the animal, that it can- 

 not but be regarded as a feature calling for generic notice, notwithstanding 

 the similarity of the soft parts. In the genus under consideration, the 



* With regard to their habits of locomotion, it must not be imagined that the gigantic 

 Strombs and Fountain Shells, with their attendant Spiders and Scorpions, are iu the habit of 

 leaping about the shore ; they are not remarkable, I conceive, for any such activity ; the word 

 "leap" must be understood in a very restricted sense, and only so far as may be necessary to 

 explain a mode of progression differing, to a limited extent, from the ordinary inert method of 

 contraction and dilatation. 



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