Mr. J. Lycett on a new species of Pteroceras. 307 



a bifurcate extensible proboscis ; the tentacula are large and di- 

 vergent, and carry the eyes at their extremities. In this family 

 are grouped the genera Strombus, Pteroceras, Rostellaria, Apor- 

 rhais, Chenopiis, and Pterodonta. 



The Strombidee are first recognised in a fossil state in the dif- 

 ferent stages of the oolitic rocks. They are more numerous in 

 the cretaceous and tertiary strata, and have attained their greatest 

 development in the present creation. They are nearly all natives 

 of tropical seas, and are most abundant in the neighbourhood 

 of coral islands. 



Pteroceras, Rostellaria and Chenopus are the most ancient 

 genera of this family ; they are still i-epresented by numerous 

 species all different from those found in a fossil state. A few 

 species of Strombus have been found in the chalk ; more have 

 been met with in the different stages of the tertiary period ; but 

 this genus has attained its full development in the seas of the 

 wai*m regions of our time, where the species are remarkable for 

 their gigantic size, singular forms, and rich and varied colour- 

 ing, Pterodonta has been found only in the chalk. Soon after 

 Lamarck had formed the new genera of his family des Ailees, 

 De Montfort* proposed his genus Hippocrena for the species 

 included by Lamarck in the genus Rostellaria which had the 

 labrum simple and dilated, the columella callous and forming a 

 channel conjointly with the labrum, which ascends close to the 

 volutions of the spire almost to its apex, the external lip with a 

 simple straight wing inflected towards the base, and with a short 

 pointed canal. He cited Rostellaria rnacroptera from the Barton 

 clay as the type of his new genus, which however has not been 

 adopted. 



Philippif recognised anatomical differences between the ani- 

 mal of Rostellaria curvirostris and that known as R. pes-pelicani ; 

 the latter has the eyes situated sessile on the sides of the tenta- 

 cula, while in the former they are terminal and retractile ; these 

 with other zootomical characters induced him to propose the ge- 

 nus Chenopus for R. pes-pelicani and other allied species : it is 

 just to observe, however, that Aldrovandus in 1623 described this 

 typical species under the generic name Aporrhais, which is now 

 adopted by British naturalists. 



The living forms of Chenopus have the respiratory canal de- 

 pressed and slightly channelled, and the labrum strongly digi- 

 tated, whilst in Rostellaria the respiratory canal is much grooved 

 and arched backwards, and the digitations when present are for 

 the most part long, slender and flexuous. Many fossil shells 



* Conchyliologie Systematique, tome ii. p. 523. 

 t Enumer. MoUuscarum Sicilise, 



20* 



